Conotoxin-sensitive and conotoxin-resistant Ca2+ currents in fish retinal ganglion cells

Curcumin pretreatment induces Nrf2 and an antioxidant response and prevents hemin-induced toxicity in primary cultures of cerebellar granule neurons of rats

Curcumin is a bifunctional antioxidant derived from Curcuma longa. This study identifies curcumin as a neuroprotectant against hemin-induced damage in primary cultures of cerebellar granule neurons (CGNs) of rats. Hemin, the oxidized form of heme, is a highly reactive compound that induces cellular injury. Pretreatment of CGNs with 5–30 μM curcumin effectively increased by 2.3–4.9 fold heme oxygenase-1 (HO-1) expression and by 5.6–14.3-fold glutathione (GSH) levels. Moreover, 15 μM curcumin attenuated by 55% the increase in reactive oxygen species (ROS) production, by 94% the reduction of GSH/glutathione disulfide (GSSG) ratio, and by 49% the cell death induced by hemin. The inhibition of heme oxygenase system or GSH synthesis with tin mesoporphyrin and buthionine sulfoximine, respectively, suppressed the protective effect of curcumin against hemin-induced toxicity. These data strongly suggest that HO-1 and GSH play a major role in the protective effect of curcumin. Furthermore, it was found that 24 h of incubation with curcumin increases by 1.4-, 2.3-, and 5.2-fold the activity of glutathione reductase, glutathione S-transferase and superoxide dismutase, respectively. Additionally, it was found that curcumin was capable of inducing nuclear factor (erythroid-derived 2)-like 2 (Nrf2) translocation into the nucleus. These data suggest that the pretreatment with curcumin induces Nrf2 and an antioxidant response that may play an important role in the protective effect of this antioxidant against hemin-induced neuronal death.

The inhibitory effect of nilvadipine on calcium channels in retinal ganglion cells in goldfish

PURPOSE

Our aim was to examine the inhibitory effect of nilvadipine on voltage-gated calcium (Ca) channels in solitary ganglion cells.

METHODS

Eyes were excised from goldfish. Ganglion cells were enzymatically dissociated from isolated retina. Whole-cell currents were recorded with the perforated-patch clamp technique.

RESULTS

Depolarizing step pulses to more than -48 mV evoked a slowly inactivating inward Ca current. The current-voltage relation for the nilvadipine-sensitive current was bellshaped, and the peak current reached a maximum at -8 mV in the presence and absence of nilvadipine. Nilvadipine block of voltage-gated Ca current was dose-dependent between 1 and 100 microM. The half-maximum inhibitory dose was 35 microM.

CONCLUSIONS

The inhibitory effect of orally administered nilvadipine on Ca channels had a mild influence in ganglion cells.

Voltage-gated currents of tilapia prolactin cells

The first recordings of neuron-like electrical activity from endocrine cells were made from fish pituitary cells. However, patch-clamping studies have predominantly utilized mammalian preparations. This study used whole-cell patch-clamping to characterize voltage-gated ionic currents of anterior pituitary cells of Oreochromis mossambicus in primary culture. Due to their importance for control of hormone secretion we emphasize analysis of calcium currents (I(Ca)), including using peptide toxins diagnostic for mammalian neuronal Ca(2+) channel types. These appear not to have been previously tested on fish endocrine cells. In balanced salines, inward currents consisted of a rapid TTX-sensitive sodium current and a smaller, slower I(Ca); there followed outward potassium currents dominated by delayed, sustained TEA-sensitive K(+) current. About half of cells tested from a holding potential (V(h)) of -90 mV showed early transient K(+) current; most cells showed a small Ca(2+)-mediated outward current. I-V plots of isolated I(Ca) with 15 mM [Ca(2+)](o) showed peak currents (up to 20 pA/pF from V(h) -90 mV) at approximately +10 mV, with approximately 60% I(Ca) for V(h) -50 mV and approximately 30% remaining at V(h) -30 mV. Plots of normalized conductance vs. voltage at several V(h)s were nearly superimposable. Well-sustained I(Ca) with predominantly Ca(2+)-dependent inactivation and inhibition of approximately 30% of total I(Ca) by nifedipine or nimodipine suggests participation of L-type channels. Each of the peptide toxins (omega-conotoxin GVIA, omega-agatoxin IVA, SNX482) alone blocked 36-54% of I(Ca). Inhibition by any of these toxins was additive to inhibition by nifedipine. Combinations of the toxins failed to produce additive effects. I(Ca) of up to 30% of total remained with any combination of inhibitors, but 0.1mM cadmium blocked all I(Ca) rapidly and reversibly. We did not find differences among cells of differing size and hormone content. Thus, I(Ca) is carried by high voltage-activated Ca(2+) channels of at least three types, but the molecular types may differ from those characterized from mammalian neurons.

Differential expression of voltage-activated calcium currents in zebrafish retinal ganglion cells

We report a study on the characterization of voltage-activated calcium currents (I(Ca)) in retinal ganglion cells (RGCs) and the topographic distribution of RGCs that express different types of I(Ca) in zebrafish retinas. In acutely isolated zebrafish RGCs, both high-voltage-activated (HVA; peak activation potential +7.4 +/- 1.1 mV) and low-voltage-activated (LVA; peak activation potential -33.0 +/- 1.2 mV) I(Ca) were recorded. HVA I(Ca) were recorded in all of the tested RGCs, whereas LVA I(Ca) were recorded in approximately one-third of the tested cells. In RGCs that expressed both HVA and LVA I(Ca), the two currents were readily separated by depolarizing the cell membrane to different voltages from different holding potentials. Among RGCs that expressed LVA I(Ca), some cells expressed large LVA I(Ca) (up to 130 pA), whereas others expressed small LVA I(Ca) (approximately 20 pA). RGCs that expressed large and small LVA I(Ca) were designated as class I and class II cells, respectively, and RGCs that expressed only HVA I(Ca) were designated as class III cells. The topographic distribution of cell classes was similar in various areas of the retina. In the nasal-ventral retina, for example, class III cells outnumbered class I and class II cells by 10.8- and 2.6-fold, respectively. In the temporal and dorsal retinas, the density of class III cells slightly decreased, whereas the density of class I and class II cells increased. The differential expression of I(Ca) in RGCs may correlate with the development and function of the retina.

Availability of low-threshold Ca2+ current in retinal ganglion cells

Spiking in central neurons depends on the availability of inward and outward currents activated by depolarization and on the activation and priming of currents by hyperpolarization. Of these processes, priming by hyperpolarization is the least described. In the case of T-type Ca2+ current availability, the interplay of hyperpolarization and depolarization has been studied most completely in expression systems, in part because of the difficulty of pharmacologically separating the Ca2+ currents of native neurons. To facilitate understanding of this current under physiological conditions, we measured T-type current of isolated goldfish retinal ganglion cells with perforated-patch voltage-clamp methods in solutions containing a normal extracellular Ca2+ concentration. The voltage sensitivities and rates of current activation, inactivation, deactivation, and recovery from inactivation were similar to those of expressed alpha1G (CaV3.1) Ca2+ channel clones, except that the rate of deactivation was significantly faster. We reproduced the amplitude and kinetics of measured T currents with a numerical simulation based on a kinetic model developed for an alpha1G Ca2+ channel. Finally, we show that this model predicts the increase of T-type current made available between resting potential and spike threshold by repetitive hyperpolarizations presented at rates that are within the bandwidth of signals processed in situ by these neurons.

Adenosine inhibits calcium channel currents via A1 receptors on salamander retinal ganglion cells in a mini-slice preparation

The effects of adenosine on high-voltage-activated calcium channel currents in tiger salamander retinal ganglion cells were investigated in a mini-slice preparation. Adenosine produced a concentration-dependent decrease in the amplitude of calcium channel current with a maximum inhibition of 26%. The effects of adenosine on calcium channel current were both time- and voltage-dependent. In cells dialyzed with GTP-gamma-s, adenosine caused a sustained and irreversible inhibition of calcium channel current, suggesting involvement of a GTP-binding protein. The inhibitory effect of adenosine on calcium channel current was blocked by the A1 antagonist 8-cyclopentyltheophylline (DPCPX, 1-10 microm), but not by the A2 antagonist 3-7-dimethyl-1-propargylxanthine (DMPX, 10 microm), and was mimicked by the A1 agonist N6-cyclohexyladenosine (CHA, 1 microm) but not by the A2 agonist 5'-(N-cyclopropyl) carbox-amidoadenosine (CPCA, 1 microm). Adenosine's inhibition of calcium channel current was not affected by the L-type calcium channel blocker nifedipine (5 microm). However, adenosine's inhibition of calcium channel current was reduced to approximately 10% after application of omega-conotoxin GVIA (1 microm), suggesting that adenosine inhibits N-type calcium channels. These results show that adenosine acts on an A1 adenosine receptor subtype via a G protein-coupled pathway to inhibit the component of calcium channel current carried in N-type calcium channels.

Heterogeneous intrinsic firing properties of vertebrate retinal ganglion cells

Retinal ganglion cells (RGCs) use their characteristic firing patterns to encode various aspects of visual information and carry them to the brain. It has been thought that the firing pattern of an RGC's light response is determined primarily by the time course and spatiotemporal interaction of the synaptic inputs. However, it is unclear whether there is a difference in intrinsic firing properties among RGCs that could contribute to the cell-to-cell distinction of the light response firing pattern. We investigated the intrinsic firing properties of isolated goldfish RGCs, minimizing cytoplasmic disturbance with a perforated-patch, whole-cell recording technique. In response to a 1-s depolarizing current step, the majority of the examined RGCs (n = 84) displayed sustained firing that lasted over 800 ms (n = 24; tonic RGCs) or transient firing accommodated within 200 ms of the step onset (n = 47; phasic RGCs). Tonic and phasic RGCs also differed in their firing frequency-current intensity dynamics. There was a significant difference in the soma sizes of phasic and tonic RGCs, indicating that some parts of these groups originate from distinct morphological subtypes. In the presence of extracellular Ba(2+) (1 mM), phasic RGCs displayed sustained firing and firing frequency-current intensity dynamics similar to those of tonic RGCs. Thus a Ba(2+)-sensitive ion current (I(Ba-s)) underlies the firing characteristics of phasic RGCs. Under voltage-clamp conditions, I(Ba-s) was identified as a low-threshold, noninactivating voltage-dependent K(+) current. Because of its slow kinetics (time constant of activation, approximately 100 ms), I(Ba-s) may confer a gradually increasing hyperpolarizing driving force during maintained excitatory stimulus, which eventually would result in firing accommodation. These findings suggest that RGCs have heterogeneous intrinsic firing properties that could aid synaptic inputs in shaping light responses.

Voltage-gated potassium channels in retinal ganglion cells of trout: a combined biophysical, pharmacological, and single-cell RT-PCR approach

Retinal ganglion cells of young mature trout were acutely isolated by tissue printing and analyzed with the whole-cell mode of the patch-clamp technique in combination with single-cell RT-PCR. All cells either exhibited spontaneous spiking activity or could be induced to fire trains of action potentials by current injection. Depolarizing voltage steps elicited a TTX-sensitive sodium inward current and a complex outward current that could be subdivided into a calcium-dependent component that was sensitive to 100 nM iberiotoxin as well as three major types of voltage-sensitive currents: 1) a high-threshold (-20 mV) noninactivating current that was highly sensitive to submicromolar TEA and quinine, resembling recombinant mammalian Kv3.1 channels; 2) a low-threshold DTX-sensitive current, matching mammalian Kv1; and 3) a fast-inactivating transient current that was highly sensitive to TEA (3 mM) but resistant to alpha-DTX (1 microM) and quinine (0.1 mM). By multiplex single-cell RT-PCR, coexpression of multiple transcripts encoding Shaker-related channel genes of trout (termed Tsha1-Tsha4) as well as two Shaw-related channels (termed Traw1 and Traw2) could be demonstrated in individual cells.

Developmental switch in excitability, Ca(2+) and K(+) currents of retinal ganglion cells and their dendritic structure

In the retina of teleost fish, continuous neuronal development occurs at the margin, in the peripheral growth zone (PGZ). We prepared tissue slices from the retina of rainbow trout that include the PGZ and that comprise a time line of retinal development, in which cells at progressive stages of differentiation are present side by side. We studied the changes in dendritic structure and voltage-dependent Ca(2+), Na(+), and K(+) currents that occur as ganglion cells mature. The youngest ganglion cells form a distinct bulge. Cells in the bulge have spare and short dendritic trees. Only half express Ca(2+) currents and then only high-voltage-activated currents with slow inactivation (HVAslow). Bulge cells are rarely electrically excitable. They express a mixture of rapidly inactivating and noninactivating K(+) currents (IKA and IKdr). The ganglion cells next organize into a transition zone, consisting of a layered structure two to three nuclei thick, before forming the single layered structure characteristic of the mature retina. In the transition zone, the dendritic arbor is elaborately branched and extends over multiple laminae in the inner plexiform layer, without apparent stratification. The arbor of the mature cells is stratified, and the span of the dendritic arbor is well over five times the cell body's diameter. The electrical properties of cells in the transition and mature zones differ significantly from those in the bulge cells. Correlated with the more elaborate dendritic structures are the expression of both rapidly inactivating HVA (HVAfast) and of low-voltage-activated (LVA) Ca(2+) currents and of a high density of Na(+) currents that renders the cells electrically excitable. The older ganglion cells also express a slowly activating K(+) current (IKsa).

Deactivation, recovery from inactivation, and modulation of extra-synaptic ion currents in fish retinal ganglion cells

As is shown magnificently by Heron Island's reef, the visual environment of many fishes includes various light intensities, hues and shapes that can change on large and small scales in space and time. Several articles in this issue address why fishes are sensitive to some of these properties, and how fishes and other aquatic species have acquired or fostered these sensitivities. This article discusses contributions of extrasynaptic ion currents, in a specific population of neurons, to the detection of ambient light levels, the appearance of certain visual stimuli and the disappearance of others.

Functional differentiation of ganglion cells from multipotent progenitor cells in sliced retina of adult goldfish

Multipotent progenitor cells at the retinal margin of adult goldfish give rise to all cell types in the rest of the retina. We took advantage of this spatial arrangement of progenitor and mature cells in slices of peripheral retina, to investigate the appearance and maturation of voltage-activated Na(+) current. We divided the peripheral retina into three broad regions (marginal, intermediate, and mature) on the basis of their morphological development. Whole-cell patch-clamp recordings were performed in ruptured-patch mode, so that cells from which currents were recorded could be identified by Lucifer Yellow fills. No voltage-activated Na(+) current was detected in the slender, peripherally located marginal cells. Voltage-activated Na(+) currents were detected in rounded cells found alongside or near marginal cells, facing the vitreal side of the retina. Some of these "intermediate cells" had a long axon-like process which ran along the vitreal surface. Intermediate cells adjacent to the marginal region tended to have smaller Na(+) currents than intermediate cells closer to the mature region. On average, the maximum Na(+) current amplitude recorded from intermediate cells was roughly 6-fold smaller than that of mature ganglion cells. In addition, the activation threshold of the Na(+) current in intermediate cells was nearly 14 mV more positive than that of mature ganglion cells. The results indicate that voltage-activated Na(+) current, as a possible marker of retinal ganglion cells, begins to develop well before these cells migrate to their adult position within the retina.

Adenosine A1 and class II metabotropic glutamate receptors mediate shared presynaptic inhibition of retinotectal transmission

Presynaptic inhibition is one of the major control mechanisms in the CNS. Previously we reported that adenosine A1 receptors mediate presynaptic inhibition at the retinotectal synapse of goldfish. Here we extend these findings to metabotropic glutamate receptors (mGluRs) and report that presynaptic inhibition produced by both A1 adenosine receptors and group II mGluRs is due to G(i) protein coupling to inhibition of N-type calcium channels in the retinal ganglion cells. Adenosine (100 microM) and an A1 (but not A2) receptor agonist reduced calcium current (I(Ca2+)) by 16-19% in cultured retinal ganglion cells, consistent with their inhibition of retinotectal synaptic transmission (-30% amplitude of field potentials). The general metabotropic glutamate receptor (mGluR) agonist 1S,3R-1-amino-cyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD, 50 microM) and the selective group II mGluR receptor agonist (2S, 2'R,3'R)-2-(2',3'-dicarboxy-cyclopropyl)glycine (DCG-IV, 300 nM) inhibited both synaptic transmission and I(Ca2+), whereas the group III mGluR agonist L-2-amino-4-phosphono-butyrate (L-AP4) inhibited neither synaptic transmission nor I(Ca2+). When the N-type calcium channels were blocked with omega-conotoxin GVIA, both adenosine and DCG-IV had much smaller percentage effects on the residual 20% of I(Ca2+), suggesting effects mainly on the N-type calcium channels. The inhibitory effects of A1 adenosine receptors and mGluRs were both blocked by pertussis toxin, indicating that they are mediated by either G(i) or G(o). They were also inhibited by activation of protein kinase C (PKC), which is known to phosphorylate and inhibit G(i). Finally, when applied sequentially, inhibition by adenosine and DCG-IV were not additive but occluded each other. Together these results suggest that adenosine A1 receptors and group II mGluRs mediate presynaptic inhibition of retinotectal synaptic transmission by sharing a pertussis toxin (PTX)-sensitive, PKC-regulated G(i) protein coupled to N-type calcium channels.

Voltage-activated calcium currents in rat retinal ganglion cells in situ: changes during prenatal and postnatal development

Voltage-activated calcium currents (ICa) are one way by which calcium influx into neurons is mediated. To investigate changes in kinetic properties of ICa during neuronal development and to correlate possible kinetic changes with specific differentiation processes, the ICa of retinal ganglion cells (RGCs) was recorded with the perforated patch-clamp technique in rat retinal slices and in whole mounts at different prenatal and postnatal stages. ICa density increased between embryonic day (E) 20 and the adult stage, paralleled by a shift in activation of the omega-conotoxin GVIA-sensitive ICa toward more negative membrane potentials. Furthermore, developmental alterations were observed in ICa inactivation rate during a 120 msec test pulse and in steady-state inactivation of ICa. The most striking feature in ICa kinetics was a transient slowing of calcium current deactivation, which peaked at postnatal day (P)3-5 and affected all ICa subtypes. Although the shift in activation and the decreased inactivation rate of ICa can be explained by differential regulation of distinct calcium channel subtypes, it is more likely that a more general alteration of the cells' functional state was the underlying factor in alterations in steady-state inactivation and current deactivation of ICa. Alterations in the omega-conotoxin GVIA-sensitive and the toxin-resistant currents temporarily coincide with dendritic differentiation, and it is tempting to speculate about their role in network formation in the inner retina. In contrast, alterations in steady-state inactivation and current deactivation may be involved in the regulation of RGC survival, because they occur during the period of programmed cell death in the ganglion cell layer. In conclusion, distinct time windows of alterations in calcium channel properties were found, and this study has provided a basis for performing functional assays to clarify in detail the developmental process to which these alterations are related.

Expression patterns of voltage-dependent calcium channel alpha 1 subunits (alpha 1A-alpha 1E) mRNA in rat retina

The transcript levels of the genes encoding for the different alpha1 (alpha1A-alpha1E) subunits of voltage-dependent calcium channels (VDCCs) were studied in the retina of the rat using RT-PCR, Northern blotting, and in situ hybridization. Abundant expression of alpha1A and alpha1B was found with RT-PCR and on Northern blots of total retina RNA, corresponding with high expression levels in all nuclear layers (outer and inner nuclear layers and the ganglion cell layer) of the retina. VDCC alpha1D mRNA was also present in all nuclear layers of the retina but at less abundant levels than alpha1A or alpha1B. Expression level of alpha1C in the retina was low as deduced from a faint Northern blot signal and a moderate yield after PCR amplification. VDCC alpha1E specific amplification of retinal cDNA yielded a longer product (designated alpha1E-L) than obtained from the hippocampus. Nucleotide sequencing of this PCR product revealed a 129 bp insert which is largely homologous (97%) with a previously described insert in the same position in human alpha1E cDNA. In situ hybridization in rat brain showed a differential expression pattern of the long and short variants of alpha1E mRNA. Northern blotting of retinal RNA confirmed the absence of the short variant (alpha1E-S), while alpha1E-L was present at low levels. In situ hybridization detected a significant level of expression of alpha1E-L in the inner nuclear layer. The prevalent expression of alpha1A and alpha1B, and to a lesser extent, of alpha1D, indicates that P/Q-, N-, and L-type calcium currents play a prominent role in the various cell types involved in the retinal signal-transduction pathway. The absence of alpha1C transcript in the retina suggests that the slowly inactivating L-type calcium currents involved in neurotransmitter release from the terminals of photoreceptors and bipolar cells may be encoded by the alpha1D isoform.

Adenosine A1 receptors mediate retinotectal presynaptic inhibition: uncoupling by C-kinase and role in LTP during regeneration

Presynaptic adenosine receptors inhibit transmitter release at many synapses and are known to exist on retinotectal terminals. In this paper we show that adenosine decreases retinotectal field potentials by approximately 30% and investigate the mechanism. First, as judged by the effects of specific calcium channel blockers, retinotectal transmission was mediated almost exclusively by N-type calcium channels, which are known to be modulated by adenosine A1 receptors. Transmission was completely blocked by either omega-Conotoxin GVIA (-100%, N-type blocker) or omega-Conotoxin MVIIC (-99%, N-, P- and Q-type blocker) and was not significantly affected by omega-Agatoxin IVA [+1.7 +/- 9. 3% (SE), P-,Q-type blocker], but was augmented slightly by nifedipine(+9.3 +/- 2.1%, L-type blocker). Second, the adenosine inhibition was presynaptic, as indicated by a 43% increase in paired-pulse facilitation. Third, the selective A1 agonist cyclohexyl adenosine (CHA) at 50 nM caused a 21% decrease in amplitude and the selective A2 agonist N6-[2-(3, 5-dimethoxyphenyl)-2-(2-methylphenyl)-ethyl]adenosine (DPMA) at 100 nM caused a 24% increase. Fourth, the selective A1 antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) alone produced an increase in the field potential, suggesting a tonic inhibition mediated by endogenous adenosine. Fifth, pertussis toxin eliminated adenosine inhibition implicating Gi or Go protein coupling. Sixth, C-kinase activation eliminated the A1-mediated inhibition. In regenerating projections, adenosine also caused a decrease in transmission (-30 +/- 12%), but after induction of long-term potentiation (LTP) via trains of stimuli or via treatment with the phosphatase inhibitor okadaic acid, the adenosine response was converted to an augmentation. Because LTP is associated with C-kinase activation, this is consistent with C-kinase uncoupling the A1 receptor from inhibiting N-type Ca2+ channels. This uncovers the A2-mediated augmentation as demonstrated in normals with DPMA. Such an effect could account in part for the LTP of immature synapses and the change from rapidly fatiguing to robust synaptic transmission.

Omega-conotoxin-MVIID blocks an omega-conotoxin-GVIA-sensitive, high-threshold Ca2+ current in fish retinal ganglion cells

Reduction of Ca2+ current amplitude by the Conus peptide omega-conotoxin-MVIID (omega-CTx-MVIID) was measured in voltage-clamped, goldfish retinal ganglion cells. Effects of depolarizing shifts in holding potential, and sequential applications of omega-CTx-MVIID, omega-CTx-GVIA, and BAY-K-8644, together with effects of Ni2+ and omega-Aga-IIIA, indicated that omega-CTx-MVIID may target Ca2+ channels differing from those termed T, L, N, P < Q and R.