Calcium-sensitive calcium influx in photoreceptor inner segments

Preservation of endoplasmic reticulum (ER) Ca2+ stores by deletion of inositol-1,4,5-trisphosphate receptor type 1 promotes ER retrotranslocation, proteostasis, and protein outer segment localization in cyclic nucleotide-gated channel-deficient cone photoreceptors

Endoplasmic reticulum (ER) Ca²⁺ homeostasis relies on an appropriate balance between efflux- and influx-channel activity responding to dynamic changes of intracellular Ca²⁺ levels. Dysregulation of this complex signaling network has been shown to contribute to neuronal and photoreceptor death in neuro- and retinal degenerative diseases, respectively. In mice with cone cyclic nucleotide-gated (CNG) channel deficiency, a model of achromatopsia/cone dystrophy, cones display early-onset ER stress-associated apoptosis and protein mislocalization. Cones in these mice also show reduced cytosolic Ca²⁺ level and subsequent elevation in the ER Ca²⁺ -efflux-channel activity, specifically the inositol-1,4,5-trisphosphate receptor type 1 (IP₃ R1), and deletion of IP₃ R1 results in preservation of cones. This work investigated how preservation of ER Ca²⁺ stores leads to cone protection. We examined the effects of cone specific deletion of IP₃ R1 on ER stress responses/cone death, protein localization, and ER proteostasis/ER-associated degradation. We demonstrated that deletion of IP₃ R1 improves trafficking of cone-specific proteins M-/S-opsin and phosphodiesterase 6C to cone outer segments and reduces localization to cone inner segments. Consistent with the improved protein localization, deletion of IP₃ R1 results in increased ER retrotranslocation protein expression, reduced proteasome subunit expression, reduced ER stress/cone death, and reduced retinal remodeling. We also observed the enhanced ER retrotranslocation in mice that have been treated with a chemical chaperone, supporting the connection between improved ER retrotranslocation/proteostasis and alleviation of ER stress. Findings from this work demonstrate the importance of ER Ca²⁺ stores in ER proteostasis and protein trafficking/localization in photoreceptors, strengthen the link between dysregulation of ER Ca²⁺ homeostasis and ER stress/cone degeneration, and support an involvement of improved ER proteostasis in ER Ca²⁺ preservation-induced cone protection; thereby identifying IP₃ R1 as a critical mediator of ER stress and protein mislocalization and as a potential target to preserve cones in CNG channel deficiency.

Ryanodine Receptor 2 Contributes to Impaired Protein Localization in Cyclic Nucleotide-Gated Channel Deficiency

The photoreceptor cyclic nucleotide-gated (CNG) channel plays a pivotal role in phototransduction and cellular calcium homeostasis. Mutations in the cone photoreceptor CNG channel subunits CNGA3 and CNGB3 are associated with achromatopsia and cone dystrophies. CNG channel deficiency leads to endoplasmic reticulum (ER) stress-associated cone apoptosis, protein mislocalization, and ER calcium dysregulation. This work investigated the potential mechanisms of protein mislocalization associated with ER calcium dysregulation using Cnga3-/- mice lacking ER Ca2+ channel ryanodine receptor 2 (RyR2) specifically in cones. Deletion of Ryr2 improved outer segment (OS) localization of the cone proteins M-opsin, S-opsin, and cone phosphodiesterase subunit α' (PDE6C) and decreased inner segment localization. One-month-old Cnga3-/- mice showed ∼30% of M-opsin, 55% of S-opsin, and 50% of PDE6C localized to the OS. Cnga3-/- mice with Ryr2 deletion at the same age showed almost 60% of M-opsin, 70% of S-opsin, and 70% of PDE6C localized to the OS. Deletion of Ryr2 nearly completely reversed elevations of the ER stress markers phospho-IRE1α and phospho-eIF2α and suppressed cone apoptosis. Consistent with the improved cone protein localization and reduced ER stress/cone apoptosis, cone survival was improved by deletion of Ryr2 The number of cones was increased by ∼28% in 2- to 4-month-old Cnga3-/- mice with Ryr2 deletion compared with age-matched Cnga3-/- mice. This work demonstrates a role of RyR2/ER calcium dysregulation in protein mislocalization, ER stress, and cone death. The findings provide novel insights into the mechanisms of photoreceptor degeneration and support strategies targeting ER calcium regulation to manage retinal degeneration.

cGMP/Protein Kinase G Signaling Suppresses Inositol 1,4,5-Trisphosphate Receptor Phosphorylation and Promotes Endoplasmic Reticulum Stress in Photoreceptors of Cyclic Nucleotide-gated Channel-deficient Mice

Photoreceptor cyclic nucleotide-gated (CNG) channels play a pivotal role in phototransduction. Mutations in the cone CNG channel subunits CNGA3 and CNGB3 are associated with achromatopsia and cone dystrophies. We have shown endoplasmic reticulum (ER) stress-associated apoptotic cone death and increased phosphorylation of the ER Ca(2+) channel inositol 1,4,5-trisphosphate receptor 1 (IP3R1) in CNG channel-deficient mice. We also presented a remarkable elevation of cGMP and an increased activity of the cGMP-dependent protein kinase (protein kinase G, PKG) in CNG channel deficiency. This work investigated whether cGMP/PKG signaling regulates ER stress and IP3R1 phosphorylation in CNG channel-deficient cones. Treatment with PKG inhibitor and deletion of guanylate cyclase-1 (GC1), the enzyme producing cGMP in cones, were used to suppress cGMP/PKG signaling in cone-dominant Cnga3(-/-)/Nrl(-/-) mice. We found that treatment with PKG inhibitor or deletion of GC1 effectively reduced apoptotic cone death, increased expression levels of cone proteins, and decreased activation of Müller glial cells. Furthermore, we observed significantly increased phosphorylation of IP3R1 and reduced ER stress. Our findings demonstrate a role of cGMP/PKG signaling in ER stress and ER Ca(2+) channel regulation and provide insights into the mechanism of cone degeneration in CNG channel deficiency.

Developing rods transplanted into the degenerating retina of Crx-knockout mice exhibit neural activity similar to native photoreceptors

Replacement of dysfunctional or dying photoreceptors offers a promising approach for retinal neurodegenerative diseases, including age-related macular degeneration and retinitis pigmentosa. Several studies have demonstrated the integration and differentiation of developing rod photoreceptors when transplanted in wild-type or degenerating retina; however, the physiology and function of the donor cells are not adequately defined. Here, we describe the physiological properties of developing rod photoreceptors that are tagged with green fluorescent protein (GFP) driven by the promoter of rod differentiation factor, Nrl. GFP-tagged developing rods show Ca(2 +) responses and rectifier outward currents that are smaller than those observed in fully developed photoreceptors, suggesting their immature developmental state. These immature rods also exhibit hyperpolarization-activated current (Ih ) induced by the activation of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. When transplanted into the subretinal space of wild-type or retinal degeneration mice, GFP-tagged developing rods can integrate into the photoreceptor outer nuclear layer in wild-type mouse retina and exhibit Ca(2 +) responses and membrane current comparable to native rod photoreceptors. A proportion of grafted rods develop rhodopsin-positive outer segment-like structures within 2 weeks after transplantation into the retina of Crx-knockout mice and produce rectifier outward current and Ih upon membrane depolarization and hyperpolarization. GFP-positive rods derived from induced pluripotent stem (iPS) cells also display similar membrane current Ih as native developing rod photoreceptors, express rod-specific phototransduction genes, and HCN-1 channels. We conclude that Nrl-promoter-driven GFP-tagged donor photoreceptors exhibit physiological characteristics of rods and that iPS cell-derived rods in vitro may provide a renewable source for cell-replacement therapy.

Intracellular pH modulates inner segment calcium homeostasis in vertebrate photoreceptors

Neuronal metabolic and electrical activity is associated with shifts in intracellular pH (pH(i)) proton activity and state-dependent changes in activation of signaling pathways in the plasma membrane, cytosol, and intracellular compartments. We investigated interactions between two intracellular messenger ions, protons and calcium (Ca²(+)), in salamander photoreceptor inner segments loaded with Ca²(+) and pH indicator dyes. Resting cytosolic pH in rods and cones in HEPES-based saline was acidified by ∼0.4 pH units with respect to pH of the superfusing saline (pH = 7.6), indicating that dissociated inner segments experience continuous acid loading. Cytosolic alkalinization with ammonium chloride (NH₄Cl) depolarized photoreceptors and stimulated Ca²(+) release from internal stores, yet paradoxically also evoked dose-dependent, reversible decreases in [Ca²(+)](i). Alkalinization-evoked [Ca²(+)](i) decreases were independent of voltage-operated and store-operated Ca²(+) entry, plasma membrane Ca²(+) extrusion, and Ca²(+) sequestration into internal stores. The [Ca²(+)](i)-suppressive effects of alkalinization were antagonized by the fast Ca²(+) buffer BAPTA, suggesting that pH(i) directly regulates Ca²(+) binding to internal anionic sites. In summary, this data suggest that endogenously produced protons continually modulate the membrane potential, release from Ca²(+) stores, and intracellular Ca²(+) buffering in rod and cone inner segments.

Light regulation of Ca2+ in the cone photoreceptor synaptic terminal

Retinal cones are depolarized in darkness, keeping voltage-gated Ca2+ channels open and sustaining exocytosis of synaptic vesicles. Light hyperpolarizes the membrane potential, closing Ca2+ channels and suppressing exocytosis. Here, we quantify the Ca2+ concentration in cone terminals, with Ca2+ indicator dyes. Two-photon ratiometric imaging of fura-2 shows that global Ca2+ averages approximately 360 nM in darkness and falls to approximately 190 nM in bright light. Depolarizing cones from their light to their dark membrane potential reveals hot spots of Ca2+ that co-label with a fluorescent probe for the synaptic ribbon protein ribeye, consistent with tight localization of Ca2+ channels near ribbons. Measurements with a low-affinity Ca2+ indicator show that the local Ca2+ concentration near the ribbon exceeds 4 M in darkness. The high level of Ca2+ near the ribbon combined with previous estimates of the Ca2+ sensitivity of release leads to a predicted dark release rate that is much faster than observed, suggesting that the cone synapse operates in a maintained state of synaptic depression in darkness.

Depletion of calcium stores regulates calcium influx and signal transmission in rod photoreceptors

Tonic synapses are specialized for sustained calcium entry and transmitter release, allowing them to operate in a graded fashion over a wide dynamic range. We identified a novel plasma membrane calcium entry mechanism that extends the range of rod photoreceptor signalling into light-adapted conditions. The mechanism, which shares molecular and physiological characteristics with store-operated calcium entry (SOCE), is required to maintain baseline [Ca(2+)](i) in rod inner segments and synaptic terminals. Sustained Ca(2+) entry into rod cytosol is augmented by store depletion, blocked by La(3+) and Gd(3+) and suppressed by organic antagonists MRS-1845 and SKF-96365. Store depletion and the subsequent Ca(2+) influx directly stimulated exocytosis in terminals of light-adapted rods loaded with the activity-dependent dye FM1-43. Moreover, SOCE blockers suppressed rod-mediated synaptic inputs to horizontal cells without affecting presynaptic voltage-operated Ca(2+) entry. Silencing of TRPC1 expression with small interference RNA disrupted SOCE in rods, but had no effect on cone Ca(2+) signalling. Rods were immunopositive for TRPC1 whereas cone inner segments immunostained with TRPC6 channel antibodies. Thus, SOCE modulates Ca(2+) homeostasis and light-evoked neurotransmission at the rod photoreceptor synapse mediated by TRPC1.

Synaptic transmission mediated by internal calcium stores in rod photoreceptors

Retinal rod photoreceptors are depolarized in darkness to approximately -40 mV, a state in which they maintain sustained glutamate release despite low levels of calcium channel activation. Blocking voltage-gated calcium channels or ryanodine receptors (RyRs) at the rod presynaptic terminal suppressed synaptic communication to bipolar cells. Spontaneous synaptic events were also inhibited when either of these pathways was blocked. This indicates that both calcium influx and calcium release from internal stores are required for the normal release of transmitter of the rod. RyR-independent release can be evoked by depolarization of a rod to a supraphysiological potential (-20 mV) that activates a large fraction of voltage-gated channels. However, this calcium channel-mediated release depletes rapidly if RyRs are blocked, indicating that RyRs support prolonged glutamate release. Thus, the rod synapse couples a small transmembrane calcium influx with a RyR-dependent amplification mechanism to support continuous vesicle release.

Pre- and post-synaptic effects of manipulating surface charge with divalent cations at the photoreceptor synapse

Persistence of horizontal cell (HC) light responses in extracellular solutions containing low Ca2+ plus divalent cations to block Ca2+ currents (ICa) has been attributed to Ca2+-independent neurotransmission. Using a retinal slice preparation to record both ICa and light responses, we demonstrate that persistence of HC responses in low [Ca2+]o can instead be explained by a paradoxical increase of Ca2+ influx into photoreceptor terminals arising from surface charge-mediated shifts in ICa activation. Consistent with this explanation, application of Zn2+ or Ni2+ caused a hyperpolarizing block of HC light responses that was relieved by lowering [Ca2+]o. The same concentrations of Zn2+ and Ni2+ reduced the amplitude of ICa at the rod dark potential and this reduction was relieved by a hyperpolarizing shift in voltage dependence induced by lowering [Ca2+]o. Block of ICa by Mg2+, which has weak surface charge effects, was not relieved by low [Ca2+]o. Recovery of HC responses in low [Ca2+]o was assisted by enhancement of rod light responses. To bypass light stimulation, OFF bipolar cells were stimulated by steps to -40 mV applied to presynaptic rods during simultaneous paired recordings. Consistent with surface charge theory, the post-synaptic current was inhibited by Zn2+ and this inhibition was relieved by lowering [Ca2+]o. Nominally divalent-free media produced inversion of HC light responses even though rod light responses remained hyperpolarizing; HC response inversion can be explained by surface charge-mediated shifts in ICa. In summary, HC light responses modifications induced by low divalent cation solutions can be explained by effects on photoreceptor light responses and membrane surface charge without necessitating Ca2+-independent neurotransmission. Furthermore, these results suggest that surface charge effects accompanying physiological changing divalent cation levels in the synaptic cleft may provide a means for modulating synaptic output from photoreceptors.

Reciprocal Modulation of Calcium Dynamics at Rod and Cone Photoreceptor Synapses by Nitric Oxide

The abundance of nitric oxide (NO) synthesizing enzymes identified in the vertebrate retina highlight the importance of NO as a signaling molecule in this tissue. Here we describe opposing actions of NO on the rod and cone photoreceptor synapse. Depolarization-induced increases of calcium concentration in rods and cones were enhanced and inhibited, respectively, by the NO donor S-nitrosocysteine. NO suppressed calcium current in cones by decreasing the maximum conductance, whereas NO facilitated rod Ca channel activation. NO also activated a nonselective voltage-independent conductance in both rods and cones. Suppression of NO production in the intact retina with NG-nitro-l-arginine favored cone over rod driven postsynaptic signals, as would be expected if NO enhanced rod and suppressed cone synaptic activity. These findings may imply involvement of NO in regulating the strength of rod and cone pathways in the retina during different states of adaptation.

Kinetics of synaptic transfer from rods and cones to horizontal cells in the salamander retina

We examined synaptic transmission between rods or cones and horizontal cells, using perforated patch recording techniques in salamander retinal slices. Experimental conditions were established under which horizontal cells received nearly pure rod or pure cone input. The response-intensity relation for both photoreceptors and horizontal cells was described by a Michaelis-Menten function with an exponent close to 1. A dynamic model was developed for the transduction from photoreceptor voltage to postsynaptic current. The basic model assumes that: (i) photoreceptor light-evoked voltage controls Ca2+ entry according to a Boltzmann relation; (ii) the rate of glutamate release depends linearly on the voltage-gated Ca2+ current (ICa) in the synaptic terminal; (iii) glutamate concentration in the synaptic cleft reflects the balance of release and reuptake in which reuptake obeys first order kinetics; (iv) the binding of glutamate to its receptor and channel gating are fast compared with glutamate kinetics in the synaptic cleft. The good fit to the model confirms that these are the key features of synaptic transmission from rods and cones. The model accommodated changes in kinetics induced by the glutamate uptake blocker, dihydrokainate. The match between model and response was not improved by including an estimate of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor desensitization or by making glutamate uptake voltage dependent.

Reciprocal interactions between calcium and chloride in rod photoreceptors

This study used imaging and electrophysiological techniques in salamander retinal slices to correlate Ca2+ and Cl- levels in rods and thus test the hypothesis of a feedback interaction between Ca2+- and Ca2+-activated Cl- channels whereby Cl- efflux through Cl- channels can inhibit Ca2+ channels. Increasing [K+]o levels produced a concentration-dependent depolarization of rods accompanied by increases in [Ca2+]i measured with Fura-2. The voltage dependence of increases in [Ca2+]i was compared with the voltage dependence of the calcium current (ICa). [Cl-]i was measured with the dye, MEQ. Depolarization with high K+ to membrane potentials below -20 mV reduced [Cl-]i; larger depolarizations increased [Cl-]i. The Na/K/Cl cotransport inhibitor, bumetanide, shifted the apparent Cl- equilibrium potential (ECl) to more negative potentials, suggesting that this cotransporter helps establish a relatively depolarized ECl. MEQ fluorescence changes evoked by high K+ were inhibited by niflumic acid (0.1 mM), NPPB (2 microM), or replacement of Ca2+ with Ba2+, suggesting that depolarization-evoked Cl- changes result partly from stimulation of Ca2+-activated Cl- channels. Replacing >/=12 mM [Cl-]o with CH3SO4- produced a significant reduction in [Cl-]i. [Ca2+]i increases evoked by 20 or 50 mM K+ were also significantly inhibited by replacing >/=12 mM [Cl-]o with CH3SO4-. Thus modest depolarization can evoke increases in [Ca2+]i that lead to reductions in [Cl-]i, and conversely, reductions in [Cl-]i inhibit depolarization-evoked [Ca2+]i increases. These findings support the hypothesis that feedback interactions between Ca2+- and Ca2+-activated Cl- channels may contribute to the regulation of presynaptic Ca2+ currents involved in synaptic transmission from rod photoreceptors.

Calcium channels at the photoreceptor synapse

Presynaptic Ca2+ channels mediate early stages of visual information processing in photoreceptors by facilitating the release of neurotransmitter and by receiving modulatory input that alters transmission. Two types of L-type Ca2+ channels, composed of alpha1F and alpha1D subunits and having similar biophysical andpharmacological properties, appear to form the principle voltage-dependent Ca2+ influx pathways in rods and cones, respectively. The role played by these channels in neurotransmitter release at these graded potential, non-spiking synapses, has been well described. The channels mediate sustained glutamate release in darkness where the cells rest at potentials near -40 mV, and signal increases in light intensity as the cells hyperpolarize negative to this value. Synaptic modulation and integration mediated by these channels has not yet been as fully described but appears to involve GABA, nitric oxide (NO), glutamate, and dopamine. Ca2+ permeable cyclic nucleotide gated (CNG) channels appear to have supporting roles at the photoreceptor output synapse and may transduce NO signals from other cells by either directly permitting Ca2+ influx or by providing depolarizing influences that gate voltage dependent Ca2+ channels.

Inhibitory action of diltiazem on voltage-gated calcium channels in cone photoreceptors

The benzothiazepine, diltiazem, is commonly used as an inhibitor of vascular L-type Ca channels, and is a clinically important anti-anginal and antihypertensive medication. In the retina, diltiazem also inhibits cyclic-nucleotide gated (CNG) channels, including the cGMP-gated channels in photoreceptors, and has been suggested to be a neuroprotectant in an animal model of retinitis pigmentosa, a degenerative disease of photoreceptors. In contrast to CNG channels, the actions of diltiazem on photoreceptor Ca channels have not been studied. We show that D-cis-diltiazem can block Ca channels in cone photoreceptors and that the potency and efficacy of cone photoreceptor Ca channel inhibition by this drug is unconventional. Over the concentration range of 5-500 microM diltiazem, the dose response curve was biphasic with a high affinity saturation level of approximately 30% block in the 20-50 microM range (IC(50)=4.9 microM) and a low affinity saturation block (near 100%) with concentrations up to 500 microM (IC(50)=100.4 microM). The degree of block was found to be equivalent when Bay K 8644 was used to increase Ca channel current, indicating that the levels of block do not result from multiple Ca channel subtypes having differing sensitivities to diltiazem. Calcium imaging showed that the relatively low efficacy of the high-affinity Ca channel block was not due to the species of charge-carrying divalent cation nor that it was associated with dialysis of cellular contents. These data contribute to an emerging perspective that the photoreceptor Ca channel has properties unique from other L-type channels, an important consideration should these channels become a target for testing putative neuroprotective therapies.

Different effects of low Ca2+ on signal transmission from rods and cones to bipolar cells in carp retina

Modulation of signal transmission from rods, red-sensitive (R-) and green-sensitive (G-) cones to bipolar cells by lowering extracellular Ca(2+) was studied in the isolated superfused carp retina using intracellular recording techniques. Low Ca(2+) (nominally Ca(2+)-free) potentiated light responses of rod dominant ON bipolar cells (rod-ON-BCs). On the other hand, responses of cone dominant ON bipolar cells (cone-ON-BCs) driven by G-cones were dramatically decreased whereas those driven by R-cones were hardly changed in low Ca(2+). Similar effects were observed in scotopic and photopic electroretinographic (ERG) b waves, which reflect the activities of ON-BCs driven by rods and cones, respectively. IBMX (100 microM), an inhibitor of PDE, whose effects mimic those of low Ca(2+) on phototransduction, increased responses of both rod-ON-BCs and cone-ON-BCs, suggesting that the distinct effects of low Ca(2+) described above are attributable to differential modulation of signal transfer from different types of photoreceptors to BCs. Moreover, scotopic ERG P III responses, reflecting the rod activity, were potentiated both in low Ca(2+) and in the presence of IBMX (100 microM). Low Ca(2+) causes multiple changes in the outer retina, including increase of glutamate release from the photoreceptor terminal, increase of current and voltage responses of photoreceptors to light, alteration of the synaptic gain from photoreceptors to BCs and modulation of mGluR6 pathway in the rod-ON-BCs. Interplay of these changes may account for differential modulation of R-cone and G-cone driven BC responses, as well as the different effects on rod- and cone-ON-BCs.

Calcium regulation in photoreceptors

In this review we describe some of the remarkable and intricate mechanisms through which the calcium ion (Ca2+) contributes to detection, transduction and synaptic transfer of light stimuli in rod and cone photoreceptors. The function of Ca2+ is highly compartmentalized. In the outer segment, Ca2+ controls photoreceptor light adaptation by independently adjusting the gain of phototransduction at several stages in the transduction chain. In the inner segment and synaptic terminal, Ca2+ regulates cells' metabolism, glutamate release, cytoskeletal dynamics, gene expression and cell death. We discuss the mechanisms of Ca2+ entry, buffering, sequestration, release from internal stores and Ca2+ extrusion from both outer and inner segments, showing that these two compartments have little in common with respect to Ca2+ homeostasis. We also investigate the various roles played by Ca2+ as an integrator of intracellular signaling pathways, and emphasize the central role played by Ca2+ as a second messenger in neuromodulation of photoreceptor signaling by extracellular ligands such as dopamine, adenosine and somatostatin. Finally, we review the intimate link between dysfunction in photoreceptor Ca2+ homeostasis and pathologies leading to retinal dysfunction and blindness.

Zn2+ modulates light responses of color-opponent bipolar and amacrine cells in the carp retina

The effects of Zn(2+) on color-opponent bipolar cells (BCs) and amacrine cells (ACs) were studied in the isolated superfused carp retina using intracellular recording techniques. Bath-applied Zn(2+) (25 micro M) depolarized R(+)G(-)-type BCs and suppressed both depolarizing responses of these cells to red (680 nm) flashes and hyperpolarizing ones to green (500 nm) flashes. Following Zn(2+) application, G(+)R(-)-type BCs were hyperpolarized, which was accompanied by a potentiation of their depolarizing responses to green flashes and a suppression of hyperpolarizing ones to red flashes. Similar Zn(2+) effects were observed in R(+)G(-)- and G(+)R(-)-type ACs. The Zn(2+) effects persisted in the presence of picrotoxin and strychnine, suggesting that modulation by Zn(2+) of GABA and glycine receptors was unlikely involved. Using whole-cell recording techniques, it was found Ca(2+) currents in cone terminals were dose-dependently suppressed by Zn(2+), suggesting that Zn(2+) may reduce glutamate release from cone photoreceptors. Furthermore, lowering extracellular Ca(2+), a procedure that increases glutamate release from photoreceptors, exerted actions on R(+)G(-)- and G(+)R(-)-type BCs, almost opposite to the Zn(2+) effects on these two types of BCs. It is therefore postulated that the Zn(2+) effects reported in the present work may reflect a consequence of the changes in input resistances of color-opponent BCs and driving forces for their light responses resulted from the reduced glutamate release by Zn(2+).

Signals from cone photoreceptors to L-type horizontal cells are differentially modulated by low calcium in carp retina

Ca2+ plays crucial roles in both phototransduction and calcium-dependent glutamate release from the photoreceptor terminal. Modulation, by lowering extracellular Ca2+, of red-sensitive (R-) and short wavelength-sensitive (S-) cone-driven light responses of L-type horizontal cells (LHCs) was studied in the isolated superfused carp retina using intracellular recording techniques. Low Ca2+ (nominally Ca2+-free) Ringer's reduced responses of LHCs to both green (500 nm) and red (680 nm) flashes in darkness, with the former being suppressed more substantially than the latter. This differential suppression became more significant when contribution of R-cones to the green-light-induced responses was diminished by a moderate red (680 nm) background light. Application of IBMX, an inhibitor of phosphodiesterase (PDE), increased LHC responses to both red and green flashes equally, resembling the effect of low Ca2+ on phototransduction. In addition, photopic electroretinographic P III responses, reflecting the activity of cones, to red flashes were more potentiated by low Ca2+, compared to those to green flashes, whilst they were both equally potentiated by IBMX. Furthermore, low Ca2+ caused a more pronounced suppression of LHC responses to red flashes than those to green flashes in the presence of IBMX. It is postulated that reduction of LHC responses in low Ca2+ may be due to the 'saturation suppression' caused by the increased glutamate release from the photoreceptor terminal and the differential modulation may reflect a consequence of the dual action of low Ca2+ on the PDE activity in the photoreceptor outer segment and the synaptic strength between cones and LHCs.

Calcium signalling mediated by the 9 acetylcholine receptor in a cochlear cell line from the Immortomouse

We have investigated the characteristics of the alpha9 acetylcholine receptor (alpha9AChR) expressed in hair cell precursors in an immortalized cell line UB/OC-2 developed from the organ of Corti of the transgenic H-2Kb-tsA58 mouse (the Immortomouse) using both calcium imaging and whole-cell recording. Ratiometric measurements of fura-2 fluorescence revealed an increase of intracellular calcium concentration in cells when challenged with 10 µM ACh. The calcium increase was seen in 66 % of the cells grown at 39 °C in differentiated conditions. A smaller fraction (34 %) of cells grown at 33 °C in proliferative conditions responded. Caffeine (10 mM) elevated cell calcium. In the absence of caffeine, the majority of imaged cells responded only once to ACh. A small proportion (< 2 % of the total) responded with an increase in intracellular calcium to multiple ACh presentations. Pretreatment with caffeine inhibited all calcium responses to ACh. In whole-cell tight-seal recordings 10 µM ACh activated an inward, non-selective cation current. The reversal potential of the ACh-activated inward current was dependent on the extracellular calcium concentration with an estimated PCa/PNa of 80 for the alpha9 receptor at physiological calcium levels. The data indicate that ACh activates a calcium-permeable channel alpha9AChR in UB/OC-2 cells and that the channel has a significantly higher calcium permeability than other AChRs. The results indicate that the alpha9AChR may be able to elevate intracellular calcium levels in hair cells both directly and via store release.

Resistance of retinal extracellular space to Ca2+ level decrease: implications for the synaptic effects of divalent cations

Ion-sensitive microelectrodes were used to measure the variations of [Ca2+]o induced by application of low Ca2+ media in the superfused eyecup preparation of the Pseudemys turtle. The aim of the experiments was to evaluate the possibility, suggested by previous studies, that in the deep, sclerad, layers of the retina [Ca2+]o may remain high enough to sustain chemical synaptic transmission even after prolonged application of low-Ca2+ saline. It was found that, at depths of 100-200 micron from the vitreal surface, [Ca2+ ]o did not fall below 1 mM even after application for periods of 30-60 min of nominally Ca2+-free media, and it was >0.3 mM after 30-min application of media containing EGTA and with a Ca2+ concentration of 1 nM. Previous studies in isolated salamander photoreceptors have shown that a reduction of [Ca2+ ]o to 0.3-1.0 mM may result in a paradoxical increase of Ca2+ influx into synaptic terminals due to the reduced screening of negative charge on the external face of the plasma membrane. On the basis of these results, the persistence or enhancement of synaptic transmission from photoreceptors to horizontal cells observed in various retinas treated with low-Ca2+ media may be accounted for within the classical Ca2+-dependent theory of synaptic transmission without invoking a Ca2+-independent mechanism.

Divalent cations modulate glutamate receptors in retinal horizontal cells of the perch (Perca fluviatilis)

Divalent cations had two effects on concentration-response relations of glutamate induced membrane currents recorded from retinal horizontal cells. The first effect was a reduction of maximum currents. Barium, magnesium, cobalt, nickel and an increased calcium concentration caused reductions of maximum currents between 14% and 70%. The second effect of divalent cations was related to the dopamine dependent modulation of glutamate receptors in horizontal cells. The dopamine dependent enhancement of glutamate gated currents requires the presence of divalent cations besides calcium in the extracellular solution. Without such divalent cations application of dopamine caused no increase of the maximum currents induced by glutamate, and only a slight shift of the half maximal saturation concentration was observed. Addition of magnesium or barium cations in millimolar concentration was sufficient to completely restore the dopamine dependent modulation.