NMDA modulation of GABA transporter current in carp retinal horizontal cells

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.

Caffeine-induced Ca2+ oscillations in type I horizontal cells of the carp retina and the contribution of the store-operated Ca2+ entry pathway

The mechanisms of release, depletion, and refilling of endoplasmic reticulum (ER) Ca²⁺ were investigated in type I horizontal cells of the carp retina using a fluo-3-based Ca²⁺ imaging technique. Exogenous application of caffeine, a ryanodine receptor agonist, induced oscillatory intracellular free Ca²⁺ concentration ([Ca²⁺]_i) responses in a duration- and concentration-dependent manner. In Ca²⁺-free Ringer’s solution, [Ca²⁺]_i transients could also be induced by a brief caffeine application, whereas subsequent caffeine application induced no [Ca²⁺]_i increase, which implied that extracellular Ca²⁺ was required for ER refilling, confirming the necessity of a Ca²⁺ influx pathway for ER refilling. Depletion of ER Ca²⁺ by thapsigargin triggered a Ca²⁺ influx which could be blocked by the store-operated channel inhibitor 2-APB, which proved the existence of the store-operated Ca²⁺ entry pathway. Taken together, these results suggested that after being depleted by caffeine, the ER was replenished by Ca²⁺ influx via store-operated channels. These results reveal the fine modulation of ER Ca²⁺ signaling, and the activation of the store-operated Ca²⁺ entry pathway guarantees the replenishment of the ER so that the cell can be ready for response to the subsequent stimulus.

NO-dependent protective effect of VEGF against excitotoxicity on layer VI of the developing cerebral cortex

In industrialized countries, cerebral palsy affects 2.5‰ of preterm and term infants. At a neurochemical level, the massive release of glutamate constitutes a major process leading to excitotoxicity and neonatal brain lesions. Previous studies, conducted in the laboratory, revealed that, in (δ/δ)VEGF(A) transgenic mice, glutamate-induced brain lesions are exacerbated suggesting that VEGF(A) could play a protective action against excitotoxicity. Using a model of cultured cortical brain slices, the aim of the study was to characterize the central effects of VEGF against glutamate-induced excitotoxicity in neonates. Exposure of brain slices to glutamate induced a strong increase of necrotic cell death in the deep cortical layer VI and a decrease of apoptotic death in superficial layers II-IV. When administered alone, a 6-h treatment with VEGF(A) had no effect on both apoptotic and necrotic deaths. In contrast, VEGF(A) abolished the glutamate-induced necrosis observed in layer VI. While MEK and PI3-K inhibitors had no effect on the protective action of VEGF(A), L-NAME, a pan inhibitor of NOS, abrogated the effect of VEGF(A) and exacerbated the excitotoxic action of glutamate. Calcimetry experiments performed on brain slices revealed that VEGF(A) reduced the massive calcium influx induced by glutamate in layer VI and this effect was blocked by L-NAME. Neuroprotective effect of VEGF(A) was also blocked by LNIO and NPLA, two inhibitors of constitutive NOS, while AGH, an iNOS inhibitor, had no effect. Nitrite measurements, electron paramagnetic resonance spectroscopy and immunohistochemistry indicated that glutamate was a potent inducer of NO production via activation of nNOS in the cortical layer VI. In vivo administration of nNOS siRNA promoted excitotoxicity and mimicked the effects of L-NAME, LNIO and NPLA. A short-term glutamate treatment increased nNOS Ser1412 phosphorylation, while a long-term exposure inhibited nNOS/NR2B protein-protein interactions. Altogether, these findings indicate that, in deep cortical layers of mice neonates, glutamate stimulates nNOS activity. Contrasting with mature brain, NO production induced by high concentrations of glutamate is neuroprotective and is required for the anti-necrotic effect of VEGF(A).

Synaptic contribution of Ca2+-permeable and Ca2+-impermeable AMPA receptors on isolated carp retinal horizontal cells and their modulation by Zn2+

Ca(2+)-permeable and Ca(2+)-impermeable AMPA receptors are co-expressed on carp retinal horizontal cells. In the present study, we examined the synaptic contribution and Zn(2+) modulatory effect of these two AMPA receptor subtypes using whole-cell patch clamp technique. Specific Ca(2+)-permeable AMPA receptor antagonist (1-naphthyl acetyl spermine, NAS) and selective Ca(2+)-impermeable AMPA receptor blocker (pentobarbital, PB) were used to separate the glutamate-response in isolated H1 horizontal cell mediated by these two subtypes of AMPA receptors respectively. Application of 100 microM NAS substantially suppressed the current elicited by 3 mM glutamate and the remaining NAS-insensitive component was completely blocked by application of 100 microM PB. In addition, Zn(2+) had dual effects on Ca(2+)-permeable AMPA receptor-mediated current: at low concentration (10 microM), Zn(2+) potentiated the current, but at higher concentrations (100 and 1000 microM), Zn(2+) reduced the current in a dose-dependent manner. However, Zn(2+) (10, 100 and 1000 microM) failed to modulate the NAS-insensitive current mediated by Ca(2+)-impermeable AMPA receptors. Overall, our results suggest that Ca(2+)-permeable AMPA receptors contribute more to the cell's glutamate-response than Ca(2+)-impermeable AMPA receptors. Furthermore, Zn(2+) has dual effects on the Ca(2+)-permeable AMPA receptor activity without affecting Ca(2+)-impermeable AMPA receptors.