Activation of NMDA receptors linked to modulation of voltage-gated ion channels and functional implications

Spike desensitisation as a mechanism for high-contrast selectivity in retinal ganglion cells

In the vertebrate retina, several dozens of parallel channels relay information about the visual world to the brain. These channels are represented by the different types of retinal ganglion cells (RGCs), whose responses are rendered selective for distinct sets of visual features by various mechanisms. These mechanisms can be roughly grouped into synaptic interactions and cell-intrinsic mechanisms, with the latter including dendritic morphology as well as ion channel complement and distribution. Here, we investigate how strongly ion channel complement can shape RGC output by comparing two mouse RGC types, the well-described ON alpha cell and a little-studied ON cell that is EGFP-labelled in the Igfbp5 mouse line and displays an unusual selectivity for stimuli with high contrast. Using patch-clamp recordings and computational modelling, we show that a higher activation threshold and a pronounced slow inactivation of the voltage-gated Na+ channels contribute to the distinct contrast tuning and transient responses in ON Igfbp5 RGCs, respectively. In contrast, such a mechanism could not be observed in ON alpha cells. This study provides an example for the powerful role that the last stage of retinal processing can play in shaping RGC responses.

Memantine Prevents the WIN 55,212-2 Evoked Cross-Priming of Ethanol-Induced Conditioned Place Preference (CPP)

The activation of the endocannabinoid system controls the release of many neurotransmitters involved in the brain reward pathways, including glutamate. Both endocannabinoid and glutamate systems are crucial for alcohol relapse. In the present study, we hypothesize that N-methyl-D-aspartate (NMDA) glutamate receptors regulate the ability of a priming dose of WIN 55,212-2 to cross-reinstate ethanol-induced conditioned place preference (CPP). To test this hypothesis, ethanol-induced (1.0 g/kg, 10% w/v, i.p.) CPP (unbiased method) was established using male adult Wistar rats. After CPP extinction, one group of animals received WIN 55,212-2 (1.0 and 2.0 mg/kg, i.p.), the cannabinoid receptor 1 (CB1) agonist, or ethanol, and the other group received memantine (3.0 or 10 mg/kg, i.p.), the NMDA antagonist and WIN 55,212-2 on the reinstatement day. Our results showed that a priming injection of WIN 55,212-2 (2.0 mg/kg, i.p.) reinstated (cross-reinstated) ethanol-induced CPP with similar efficacy to ethanol. Memantine (3.0 or 10 mg/kg, i.p.) pretreatment blocked this WIN 55,212-2 effect. Furthermore, our experiments indicated that ethanol withdrawal (7 days withdrawal after 10 days ethanol administration) down-regulated the CNR1 (encoding CB1), GRIN1/2A (encoding GluN1 and GluN2A subunit of the NMDA receptor) genes expression in the prefrontal cortex and dorsal striatum, but up-regulated these in the hippocampus, confirming the involvement of these receptors in ethanol rewarding effects. Thus, our results show that the endocannabinoid system is involved in the motivational properties of ethanol, and glutamate may control cannabinoid induced relapse into ethanol seeking behavior.

The effects of intra-dorsal hippocampus infusion of pregnenolone sulfate on memory function and hippocampal BDNF mRNA expression of biliary cirrhosis-induced memory impairment in rats

Learning and memory impairment is one of the most challenging complications of cirrhosis and present treatments are unsatisfactory. The exact mechanism of cirrhosis cognitive dysfunction is unknown. Pregnenolone sulfate (PREGS) is an excitatory neurosteroid that acts as a N-methyl-D-aspartate (NMDA) receptor agonist and GABAA receptor antagonist. In this study we evaluated the effect of intra CA1 infusion of PREGS on cirrhotic rats' memory function using the Y-maze test. Hippocampal brain-derived neurotrophic factor (BDNF) mRNA expression was also evaluated. Three weeks after bile duct ligation (BDL) surgery, rats were under stereotaxic surgery for insertion of two guide cannulas in the CA1 region of the hippocampus. After 1-week of recovery, PREGS was administered through CA1 cannulas in cirrhotic rats, while control or sham groups received vehicle. For evaluation of NMDA receptor role in memory-enhancing effects of PREGS, DL-2-Amino-5-phosphonopentanoic acid (AP5) which is a potent and competitive antagonist of NMDA receptor, co-administered with PREGS and for assessment of hippocampal BDNF mRNA expression, quantitative Real-time reverse transcriptase-PCR (RT-PCR) was used. Results showed that 28 days after BDL, cirrhotic animals' memory significantly decreased in comparison with control and sham groups, while PREGS infusion could restore memory impairment (P<0.05). PREGS effects on memory of cirrhotic rats were antagonized by DAP5. RT-PCR findings have shown that hippocampal relative BDNF mRNA expression was up-regulated in PREGS-treated groups in comparison with the BDL group (P<0.001). Our findings suggest that PREGS has a memory-enhancing effect in cirrhosis memory deficit in acute therapy and this effect may be through NMDA (glutamate) receptor involvement and BDNF mRNA expression.

Tropisetron as a neuroprotective agent against glutamate-induced excitotoxicity and mechanisms of action

The objective of this study was to determine the neuroprotective role of tropisetron on retinal ganglion cells (RGCs) as well as to explore the possible mechanisms associated with alpha7 nAChR-induced neuroprotection. Adult pig RGCs were isolated from all other retinal tissue using a two-step panning technique. Once isolated, RGCs were cultured for 3 days under control untreated conditions, in the presence of 500 μM glutamate to induce excitotoxicity, and when tropisetron was applied before glutamate to induce neuroprotection. 500 μM glutamate decreased RGC survival by an average of 62% compared to control conditions. However, RGCs pretreated with 100 nM tropisetron before glutamate increased cell survival to an average of 105% compared to controls. Inhibition studies using the alpha7 nAChR antagonist, MLA (10 nM), support the hypothesis that tropisetron is an effective neuroprotective agent against glutamate-induced excitotoxicity; mediated by α7 nAChR activation. ELISA studies were performed to determine if signaling cascades normally associated with excitotoxicity and neuroprotection were up- or down-regulated after tropisetron treatment. Tropisetron had no discernible effects on pAkt levels but significantly decreased p38 MAPK levels associated with excitotoxicity from an average of 15 ng/ml to 6 ng/ml. Another mechanism shown to be associated with neuroprotection involves internalization of NMDA receptors. Double-labeled immunocytochemistry and electrophysiology studies provided further evidence that tropisetron caused internalization of NMDA receptor subunits. The findings of this study suggest that tropisetron could be an effective therapeutic agent for the treatment of degenerative disorders of the central nervous system that involves excitotoxicity.

PIKfyve regulates CaV1.2 degradation and prevents excitotoxic cell death

Voltage-gated Ca(2+) channels (VGCCs) play a key role in neuronal signaling but can also contribute to cellular dysfunction and death under pathological conditions such as stroke and neurodegenerative diseases. We report that activation of N-methyl-D-aspartic acid receptors causes internalization and degradation of Ca(V)1.2 channels, resulting in decreased Ca(2+) entry and reduced toxicity. Ca(V)1.2 internalization and degradation requires binding to phosphatidylinositol 3-phosphate 5-kinase (PIKfyve), a lipid kinase which generates phosphatidylinositol (3,5)-bisphosphate (PtdIns(3,5)P(2)) and regulates endosome and lysosome function. Sustained activation of glutamate receptors recruits PIKfyve to Ca(V)1.2 channels, increases cellular levels of PtdIns(3,5)P(2), and promotes targeting of Ca(V)1.2 to lysosomes. Knockdown of PIKfyve prevents Ca(V)1.2 degradation and increases neuronal susceptibility to excitotoxicity. These experiments identify a novel mechanism by which neurons are protected from excitotoxicity and provide a possible explanation for neuronal death in diseases caused by mutations that affect PtdIns(3,5)P(2) regulation.

Intrinsic light response of retinal horizontal cells of teleosts

The discovery of intrinsically photosensitive retinal ganglion cells has overthrown the long-held belief that rods and cones are the exclusive retinal photoreceptors. Intrinsically photosensitive retinal ganglion cells use melanopsin as the photopigment, and mediate non-image-forming visual functions such as circadian photoentrainment. In fish, in situ hybridization studies indicated that melanopsin is present in retinal horizontal cells-lateral association neurons critical for creating the centre-surround receptive fields of visual neurons. This raises the question of whether fish horizontal cells are intrinsically photosensitive. This notion was examined previously in flat-mount roach retina, but all horizontal-cell light response disappeared after synaptic transmission was blocked, making any conclusion difficult to reach. To examine this question directly, we have now recorded from single, acutely dissociated horizontal cells from catfish and goldfish. We found that light induced a response in catfish cone horizontal cells, but not rod horizontal cells, consisting of a modulation of the nifedipine-sensitive, voltage-gated calcium current. The light response was extremely slow, lasting for many minutes. Similar light responses were observed in a high percentage of goldfish horizontal cells. We have cloned two melanopsin genes and one vertebrate ancient (VA) opsin gene from catfish. In situ hybridization indicated that melanopsin, but less likely VA opsin, was expressed in the horizontal-cell layer of catfish retina. This intrinsic light response may serve to modulate, over a long timescale, lateral inhibition mediated by these cells. Thus, at least in some vertebrates, there are retinal non-rod/non-cone photoreceptors involved primarily in image-forming vision.

Functional N-Methyl-D-Aspartate Receptors Are Expressed in Cone-Driven Horizontal Cells in Carp Retina

Glutamate works as a major excitatory neurotransmitter in the vertebrate retina. Whole-cell recordings made from isolated carp cone horizontal cells (H1 cells) showed that N-methyl-D-aspartate (NMDA), co-applied with glycine, induced inward currents that were blocked by the NMDA receptor competitive antagonist D-2-amino-5-phosphonopentanoate (D-AP5) and 5,7-dichlorokynurenic acid (DCKA), a selective NMDA receptor antagonist acting at the glycine site on the NMDA receptor complex. Moreover, calcium imaging showed that NMDA caused a significant elevation of intracellular calcium levels ([Ca(2+)](i)) of H1 cells, which was also blocked by D-AP5. In contrast, neither inward currents nor changes in [Ca(2+)](i) could be induced by NMDA in rod horizontal cells (H4 cells). Intracellular recordings made from H1 cells in the isolated retina, superfused with Ringer's containing 1 mM Mg(2+), in the dark demonstrated that NMDA reduced the light-off overshoot of H1 cells. We therefore conclude that the functional NMDA receptor is expressed in carp H1 cells, from which this receptor has been thought to be absent, and this receptor may play a role in modulating cone-driven signal of horizontal cells in the dark.

Mechanism linking NMDA receptor activation to modulation of voltage-gated sodium current in distal retina

In this study, we investigated the mechanism that links activation of N-methyl-D-aspartate (NMDA) receptors to inhibition of voltage-gated sodium channels in isolated catfish cone horizontal cells. NMDA channels were activated in voltage-clamped cells incubated in low-calcium saline or dialyzed with the calcium chelator BAPTA to determine that calcium influx through NMDA channels is required for sodium channel modulation. To determine whether calcium influx through NMDA channels triggers calcium-induced calcium release (CICR), cells were loaded with the calcium-sensitive dye calcium green 2 and changes in relative fluorescence were measured in response to NMDA. Responses were compared with measurements obtained when caffeine depleted stores. Voltage-clamp studies demonstrated that CICR modulated sodium channels in a manner similar to that of NMDA. Blocking NMDA receptors with AP-7, blocking CICR with ruthenium red, depleting stores with caffeine, or dialyzing cells with calmodulin antagonists W-5 or peptide 290-309 all prevented sodium channel modulation. These results support the hypothesis that NMDA modulation of voltage-gated sodium channels in horizontal cells requires CICR and activation of a calmodulin-dependent signaling pathway.