Distribution of the inositol trisphosphate receptor in the catfish retina

Calcium dynamics and regulation in horizontal cells of the vertebrate retina: lessons from teleosts

Horizontal cells (HCs) are inhibitory interneurons of the vertebrate retina. Unlike typical neurons, HCs are chronically depolarized in the dark, leading to a constant influx of Ca2+ Therefore, mechanisms of Ca2+ homeostasis in HCs must differ from neurons elsewhere in the central nervous system, which undergo excitotoxicity when they are chronically depolarized or stressed with Ca2+ HCs are especially well characterized in teleost fish and have been used to unlock mysteries of the vertebrate retina for over one century. More recently, mammalian models of the retina have been increasingly informative for HC physiology. We draw from both teleost and mammalian models in this review, using a comparative approach to examine what is known about Ca2+ pathways in vertebrate HCs. We begin with a survey of Ca2+-permeable ion channels, exchangers, and pumps and summarize Ca2+ influx and efflux pathways, buffering, and intracellular stores. This includes evidence for Ca2+-permeable α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors and N-methyl-d-aspartate receptors and for voltage-gated Ca2+ channels. Special attention is given to interactions between ion channels, to differences among species, and in which subtypes of HCs these channels have been found. We then discuss a number of unresolved issues pertaining to Ca2+ dynamics in HCs, including a potential role for Ca2+ in feedback to photoreceptors, the role for Ca2+-induced Ca2+ release, and the properties and functions of Ca2+-based action potentials. This review aims to highlight the unique Ca2+ dynamics in HCs, as these are inextricably tied to retinal function.

Carbonic anhydrase-related protein VIII is expressed in rod bipolar cells and alters signaling at the rod bipolar to AII-amacrine cell synapse in the mammalian retina

Mutation of the gene encoding carbonic anhydrase-related protein VIII (CAVIII) results in motor coordination deficits in mice and humans, due to loss of this protein in Purkinje cells of the cerebellum. Recent studies have indicated that the CAVIII gene, Car8, is also expressed in rod bipolar cells (RBCs), a critical glutamatergic neuron for scotopic vision. We investigated the localization of CAVIII in the mouse and macaque retina, and utilized the wdl mouse, which has a null mutation in the Car8 gene, to determine how the loss of CAVIII affects retinal signaling. CAVIII immunoreactivity was observed in RBCs, with particularly high staining intensity in the axon terminals. In addition, weaker staining was observed in a subset of cone bipolar cells and γ-aminobutyric acid (GABA)ergic amacrine cells. Light-evoked current and voltage responses of RBCs were not altered in the wdl mutant. However, light-evoked current responses from the AII-amacrine cell, a postsynaptic partner at the RBC ribbon synapse, were significantly larger, and more prolonged than in control mice. These changes could not be attributed to alterations in calcium current activation or inactivation, or to changes in the density of RBCs. Furthermore, no gross synaptic alterations were evident in the wdl mutant at the light or ultrastructural level. These data provide evidence that the CAVIII protein, which is highly conserved in vertebrates, is selectively expressed within neural circuits, and may be important for modulating retinal neurotransmission.

Development of new peptide-based tools for studying synaptic ribbon function

Synaptic ribbons are proteinaceous specialized electron-dense presynaptic structures found in nonspiking sensory cells of the vertebrate nervous system. Understanding the function of these structures is an active area of research (reviewed in Matthews G, Fuchs P. Nat Rev Neurosci 11: 812-822, 2010). Previous work had shown that ribbons could be effectively labeled and visualized using peptides that bind to the synaptic ribbon protein RIBEYE via a PXDLS motif (Zenisek D, Horst NK, Merrifield C, Sterling P, Matthews G. J Neurosci 24: 9752-9759, 2004). Here, we expand on the previous work to develop new tools and strategies for 1) better visualizing synaptic ribbons, and 2) monitoring and manipulating calcium on the synaptic ribbon. Specifically, we developed a new higher-affinity peptide-based label for visualizing ribbons in live cells and two strategies for localizing calcium indicators to the synaptic ribbon.

Progesterone potentiates IP(3)-mediated calcium signaling through Akt/PKB

The activity of cells critically depends on the control of their cytosolic free calcium ion (Ca(2+)) concentration. The objective of the present study was to identify mechanisms of action underlying the control of the gain of intracellular Ca(2+) release by circulating gonadal steroid hormones. Acute stimulation of isolated neurons with progesterone led to IP(3)R-mediated Ca(2+) transients that depend on the activation of the PI3 kinase/Akt/PKB signaling pathway. These results were confirmed at the molecular level and phosphorylation of IP(3)R type 1 by Akt/PKB was identified as the mechanism of action. Hence, it is likely that circulating gonadal steroid hormones control neuronal activity including phosporylation status through receptor- and kinase-mediated signaling. With a direct control of the gain of the Ca(2+) second messenger system as a signaling gatekeeper for neuronal activity the present study identifies a novel pathway for interaction of the endocrine and central nervous system.

Calcium-binding protein Caldendrin and CaMKII are localized in spinules of the carp retina

Calcium-binding proteins translate the influx of Ca(2+) at excitatory synapses into spatiotemporal signals that regulate a variety of processes underlying synaptic plasticity. In the fish retina, the synaptic connectivity between photoreceptors and horizontal cells undergoes a remarkable plasticity, triggered by the ambient light conditions. With increasing light, the synaptic dendrites of horizontal cells form numerous spinules that are dissolved during dark adaptation. The dynamic regulation of this process is calcium-dependent and involves the Ca(2+)/calmodulin-dependent protein kinase II (CaMKII), but astonishingly its principal regulator Calmodulin (CaM) could not be localized to spinules. Here, we show that antibodies directed against Caldendrin (CaBP1), a member of the EF-hand calcium-binding protein family, strongly label the terminal dendrites of horizontal cells invaginating cone pedicles. Double-labeling experiments revealed that this label is closely associated with label for CaMKII. This association was confirmed at the ultrastructural level. Caldendrin immunoreactivity and CaMKII immunoreactivity are both present in horizontal cell dendrites flanking the synaptic ribbon within the cone pedicle and in particular in spinules formed by these terminals. Comparison of light- and dark-adapted retinas revealed a shift of the membrane-associated label for Caldendrin from the terminal dendrites into the spinules during light adaptation. These results suggest that Caldendrin is involved in the dynamic regulation of spinules and confirms the assumed potential of Caldendrin as a neural calcium sensor for synaptic plasticity.

Modulation of a voltage-gated calcium channel linked to activation of glutamate receptors and calcium-induced calcium release in the catfish retina

1. Catfish (Ictalurus punctatus) retinal cone horizontal cells contain an L-type calcium current that has been proposed to be involved in visual processing. Here we report on the modulation of this current by activation of glutamate receptors and calcium-induced calcium release (CICR) from intracellular calcium stores. 2. Fluorescence data obtained from isolated horizontal cells loaded with indo-1 provided evidence of calcium release from an intracellular calcium store sensitive to caffeine, calcium and ryanodine. In the presence of caffeine, ryanodine-sensitive stores released calcium in a transient manner. Release of calcium was blocked when cells were preincubated in BAPTA, in the presence of ruthenium red, or in low concentrations of ryanodine. 3. The release of calcium from ryanodine-sensitive stores directly corresponded with a decrease of the voltage-gated L-type calcium current amplitude. Caffeine-induced modulation of the calcium current was reduced in the presence of ruthenium red. 4. Activation of ionotropic kainate receptors on catfish cone horizontal cells triggered CICR from ryanodine-sensitive stores and mimicked inhibition of the voltage-gated calcium current. Kainate-induced inhibition of the calcium current was diminished when intracellular calcium stores were inhibited with ruthenium red or depleted with ryanodine, or when calmodulin antagonists or CaM kinase II inhibitors were present. 5. These results provide evidence that activation of an ionotropic glutamate receptor on catfish cone horizontal cells is linked to calcium release from ryanodine-sensitive intracellular calcium stores and modulation of the L-type calcium current activity. Inhibition of this calcium current directly or indirectly involves calmodulin and CaM kinase II and represents a possible mechanism used by horizontal cells to affect response properties of these cells.

Regulation of transmitter release from retinal bipolar cells

Mb1 bipolar cells (ON-type cells) of the goldfish retina have exceptionally large (approximately 10 microns in diameter) presynaptic terminals, and thus, are suitable for investigating presynaptic mechanisms for transmitter release. Using enzymatically dissociated Mb1 bipolar cells under whole-cell voltage clamp, we measured the Ca2+ current (ICa), the intracellular free Ca2+ concentration ([Ca2+]i), and membrane capacitance changes associated with exocytosis and endocytosis. Release of transmitter (glutamate) was monitored electrophysiologically by a glutamate receptor-rich neuron as a probe. L-type Ca2+ channels were localized at the presynaptic terminals. The presynaptic [Ca2+]i was strongly regulated by cytoplasmic Ca2+ buffers, the Na(+)-Ca2+ exchanger and the Ca2+ pump in the plasma membrane. Once ICa was activated, a steep Ca2+ gradient was created around Ca2+ channels; [Ca2+]i increased to approximately 100 microM at the fusion sites of synaptic vesicles whereas up to approximately 1 microM at the cytoplasm. The short delay (approximately 1 ms) of exocytosis and the lack of prominent asynchronous release after the termination of ICa suggested a low-affinity Ca2+ fusion sensor for exocytosis. Depending on the rate of Ca2+ influx, glutamate was released in a rapid phasic mode as well as a tonic mode. Multiple pools of synaptic vesicles as well as vesicle cycling seemed to support continuous glutamate release. Activation of protein kinase C increased the size of synaptic vesicle pool, resulting in the potentiation of glutamate release. Goldfish Mb1 bipolar cells may still be an important model system for understanding the molecular mechanisms of transmitter release.

Calcium released from intracellular stores inhibits GABAA-mediated currents in ganglion cells of the turtle retina

We studied spiking neurons isolated from turtle retina by the whole cell version of the patch clamp. The studied cells had perikaryal diameters > 15 microns and fired multiple spikes in response to depolarizing current steps, indicating they were ganglion cells. In symmetrical [Cl-], currents elicited by puffs of 100 microM gamma-aminobutyric acid (GABA) were inward at a holding potential of -80 mV. All of the GABA-evoked current was blocked by SR95331 (20 microM), indicating that it was mediated by a GABAA receptor. The GABA-evoked currents were unaltered by eliciting a transmembrane calcium current either just before or during the response to GABA. On the other hand caffeine (10 mM), which induces Ca2+ release from intracellular stores, inhibited the GABA-evoked current on average by 30%. The caffeine effect was blocked by introducing the calcium buffer bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) into the cell but was unaffected by replacing [Ca2+]o with equimolar cobalt. Thapsigargin (10 microM), an inhibitor of intracellular calcium pumps, and ryanodine (20 microM), which depletes intracellular calcium stores, both markedly reduced a caffeine-induced inhibition of the GABA-evoked current. Another activator of intracellular calcium release, inositol trisphosphate (IP3; 50 microM), also progressively reduced the GABA-induced current when introduced into the cell. Dibutyryl adenosine 3'5'-cyclic monophosphate (cAMP; 0.5 mM), a membrane-permeable analogue of cAMP, did not reduce GABA-evoked currents, suggesting that cAMP-dependent kinases are not involved in suppressing GABAA currents, whereas calmidazolium (30 microM) and cyclosporin A (20 microM), which inhibit Ca/calmodulin-dependent phosphatases, did reduce the caffeine-induced inhibition of the GABA-evoked current. Alkaline phosphatase (150 micrograms/ml) and calcineurin (300 micrograms/ml) had a similar action to caffeine or IP3. Antibodies directed against the ryanodine receptor or the IP3 receptor reacted with the great majority of neurons in the ganglion cell layer. We found that these two antibodies colocalized in large ganglion cells. In summary, intracellular calcium plays a role in reducing the currents elicited by GABA, acting through GABAA receptors. The modulatory action of calcium on GABA responses appears to work through one or more Ca-dependent phosphatases.

Na+/Ca2+ exchange in catfish retina horizontal cells: regulation of intracellular Ca2+ store function

The role of the Na+/Ca2+ exchanger in intracellular Ca2+ regulation was investigated in freshly dissociated catfish retinal horizontal cells (HC). Ca2+-permeable glutamate receptors and L-type Ca2+ channels as well as inositol 1,4,5-trisphosphate-sensitive and caffeine-sensitive intracellular Ca2+ stores regulate intracellular Ca2+ in these cells. We used the Ca2+-sensitive dye fluo 3 to measure changes in intracellular Ca2+ concentration ([Ca2+]i) under conditions in which Na+/Ca2+ exchange was altered. In addition, the role of the Na+/Ca2+ exchanger in the refilling of the caffeine-sensitive Ca2+ store following caffeine-stimulated Ca2+ release was assessed. Brief applications of caffeine (1-10 s) produced rapid and transient changes in [Ca2+]i. Repeated applications of caffeine produced smaller Ca2+ transients until no further Ca2+ was released. Store refilling occurred within 1-2 min and required extracellular Ca2+. Ouabain-induced increases in intracellular Na+ concentration ([Na+]i) increased both basal free [Ca2+]i and caffeine-stimulated Ca2+ release. Reduction of external Na+ concentration ([Na+]o) further and reversibly increased [Ca2+]i in ouabain-treated HC. This effect was not abolished by the Ca2+ channel blocker nifedipine, suggesting that increases in [Na+]i promote net extracellular Ca2+ influx through a Na+/Ca2+ exchanger. Moreover, when [Na+]o was replaced by Li+, caffeine did not stimulate release of Ca2+ from the caffeine-sensitive store after Ca2+ depletion. The Na+/Ca2+ exchanger inhibitor 2',4'-dimethylbenzamil significantly reduced the caffeine-evoked Ca2+ response 1 and 2 min after store depletion.