Invulnerability of retinal ganglion cells to NMDA excitotoxicity

NMDA excitotoxicity has been proposed to mediate the death of retinal ganglion cells (RGCs) in glaucoma and ischemia. Here, we reexamine the effects of glutamate and NMDA on rat RGCs in vitro and in situ. We show that highly purified RGCs express NR1 and NR2 receptor subunits by Western blotting and immunostaining, and functional NMDA receptor channels by whole-cell patch-clamp recording. Nevertheless, high concentrations of glutamate or NMDA failed to induce the death of purified RGCs, even after prolonged exposure for 24 h. RGCs co-cultured together with ephrins, astrocytes, or mixed retinal cells were similarly invulnerable to glutamate and NMDA, though their NMDA currents were 4-fold larger. In contrast, even a short exposure to glutamate or NMDA induced the rapid and profound excitotoxic death of most hippocampal neurons in culture. To determine whether RGCs in an intact retina are vulnerable to excitotoxicity, we retrogradely labeled RGCs in vivo using fluorogold and exposed acutely isolated intact retinas to high concentrations of glutamate or NMDA. This produced a substantial and rapid loss of amacrine cells; however, RGCs were not affected. Nonetheless, RGCs expressed NMDA currents in situ that were larger than those reported for amacrine cells. Interestingly, the NMDA receptors expressed by RGCs were extrasynaptically localized both in vitro and in situ. These results indicate that RGCs in vitro and in situ are relatively invulnerable to glutamate and NMDA excitotoxicity compared to amacrine cells, and indicate that important, as yet unidentified, determinants downstream of NMDA receptors control vulnerability to excitotoxicity.

Neuronal glutamate transporters limit activation of NMDA receptors by neurotransmitter spillover on CA1 pyramidal cells

Glutamate released at synapses in the CA1 region of the hippocampus escapes the synaptic cleft and activates extrasynaptic targets; it also may "spill over" into neighboring synapses and activate receptors there. Glutamate transporters in glial membranes restrict extrasynaptic diffusion, but it is unclear whether neuronal glutamate transporters also limit transmitter diffusion and receptor activation by spillover. I examined the effects of a low-affinity competitive NMDA receptor antagonist on EPSCs in acute hippocampal slices to distinguish receptors activated within active synapses from those activated by spillover. Glutamate spillover is observed between Schaffer collateral fiber synapses onto CA1 pyramidal cells only when transporters in the postsynaptic neuron are inhibited. Because glutamate transporters operate most effectively at negative membrane potentials, these results suggest that activation of NMDA receptors by spillover may depend on postsynaptic activity.

Synaptically released glutamate does not overwhelm transporters on hippocampal astrocytes during high-frequency stimulation

In addition to maintaining the extracellular glutamate concentration at low ambient levels, high-affinity glutamate transporters play a direct role in synaptic transmission by speeding the clearance of glutamate from the synaptic cleft and limiting the extent to which transmitter spills over between synapses. Transporters are expressed in both neurons and glia, but glial transporters are likely to play the major role in removing synaptically released glutamate from the extracellular space. The role of transporters in synaptic transmission has been studied directly by measuring synaptically activated, transporter-mediated currents (STCs) in neurons and astrocytes. Here we record from astrocytes in the CA1 region of hippocampal slices and elicit STCs with high-frequency (100 Hz) stimulus trains of varying length to determine whether transporters are overwhelmed by stimuli that induce long-term potentiation. We show that, at near-physiological temperatures (34 degrees C), high-frequency stimulation (HFS) does not affect the rate at which transporters clear glutamate from the extrasynaptic space. Thus, although spillover between synapses during "normal" stimulation may compromise the absolute synapse specificity of fast excitatory synaptic transmission, spillover is not exacerbated during HFS. Transporter capacity is diminished somewhat at room temperature (24 degrees C), although transmitter released during brief, "theta burst" stimulation is still cleared as quickly as following a single stimulus, even when transport capacity is partially diminished by pharmacological means.

Clearance of glutamate inside the synapse and beyond

The heated debate over the level of postsynaptic receptor occupancy by transmitter has not been extinguished - indeed, new evidence is fanning the flames. Recent experiments using two-photon microscopy suggest that the concentration of glutamate in the synaptic cleft does not attain levels previously suggested. In contrast, recordings from glial cells and studies of extrasynaptic receptor activation indicate that significant quantities of glutamate escape from the cleft following exocytosis. Determining the amount of glutamate efflux from the synaptic cleft and the distance it diffuses is critical to issues of synaptic specificity and the induction of synaptic plasticity.

Glutamate release monitored with astrocyte transporter currents during LTP

Long-term potentiation (LTP) of synaptic transmission in the CA1 region of the hippocampus is thought to result from either increased transmitter release, heightened postsynaptic sensitivity, or a combination of the two. We have measured evoked glutamate release from Schaffer collateral/commissural fiber terminals in CA1 by recording synaptically activated glutamate transporter currents in hippocampal astrocytes located in stratum radiatum. Although several manipulations of release probability caused parallel changes in extracellular field potentials and synaptically activated transporter current amplitudes, induction of LTP failed to alter transporter-mediated responses, suggesting that LTP does not alter the amount of glutamate released upon synaptic stimulation.

Transporters buffer synaptically released glutamate on a submillisecond time scale

The role of transporters in clearing free glutamate from the synaptic cleft was studied in rat CA1 hippocampal neurons cultured on glial microislands. The time course of free glutamate in the cleft during a synaptic event was estimated by measuring the extent to which the rapidly dissociating AMPA receptor antagonist kynurenate (KYN) was replaced by glutamate during a synaptic response. Dose inhibition of the AMPA receptor EPSC by KYN was less than predicted by the equilibrium affinity of the antagonist, and the rise time of AMPA receptor miniature EPSCs (mEPSCs) was slowed by KYN. Both results indicated that KYN dissociated from AMPA receptors and was replaced by synaptically released transmitter. When transporters were blocked by D,L-threo-beta-hydroxyaspartic acid (THA) or Li+, the mEPSC rise time in the presence of KYN was slowed further, indicating that transporters affect the glutamate concentration in the first few hundred microseconds of the synaptic response. The glutamate transient necessary to cause these effects was determined by developing a detailed kinetic model of the AMPA receptor. The model replicated the effects of KYN on the amplitude and rise time of the synaptic responses when driven by glutamate transients that were similar to previous estimates (; ). The effects of THA were replicated by slowing and enlarging the slower phase of the dual component transient by about 20% or by prolonging the single component by almost 40%. Because transport is too slow to account for these effects, it is concluded that transporters buffer glutamate in the synaptic cleft.

Asynchronous release of synaptic vesicles determines the time course of the AMPA receptor-mediated EPSC

The contribution of intersynaptic transmitter diffusion to the AMPA receptor EPSC time course was studied in cultured CA1 hippocampal neurons. Reducing release probability 20-fold with cadmium did not affect the time course of the averaged AMPA receptor EPSC, even when receptor desensitization was blocked by cyclothiazide, suggesting that individual synapses contribute independently to the AMPA receptor-mediated EPSC. Deconvolution of the averaged miniature EPSC from the evoked EPSC showed that release probability decays only slightly faster than the EPSC, suggesting that the AMPA receptor EPSC time course is determined primarily by the asynchrony of vesicle release. Further experiments demonstrated that cyclothiazide, previously thought to affect only AMPA receptor kinetics, also enhances synaptic release.

The relationship between light-evoked synaptic excitation and spiking behaviour of salamander retinal ganglion cells

1. Light-evoked input-output characteristics of ganglion cells in dark-adapted tiger salamander retina were studied in the slice preparation using patch-clamp techniques. Excitatory postsynaptic currents (EPSCs), isolated by blocking inhibitory inputs and evoked by a range of light stimulus intensities, were recorded under whole-cell voltage clamp. Spike responses, evoked by the same light intensities, were recorded extracellularly from the same cells using the cell-attached patch-clamp technique. 2. When N-methyl-D-aspartate (NMDA) receptor-mediated input was blocked by the competitive NMDA antagonist DL-2-amino-5-phosphonoheptanoate (AP7), light-evoked EPSC amplitude and peak firing rate were reduced at all light intensities. In both cases, the data obtained in the presence of AP7 scaled linearly to control data, indicating that NMDA and non-NMDA receptors are activated in the same proportions across the entire 2 log unit stimulus response range of these ganglion cells. 3. The relationship between light-evoked spike frequency and light-evoked EPSC amplitude was linear. The slope of the light-evoked synaptic current-spike frequency relationship was close to the slope of the injected current-spike frequency relationship, indicating that synaptic current and injected current drive spiking in a similar manner. The linearity of the synaptic current-spike frequency relationship was not compromised when NMDA input was blocked by AP7. 4. Light-evoked voltage responses, recorded under whole-cell current clamp, revealed that the average membrane potential during a spike response was depolarized only slightly with increased firing rate. Once the membrane potential surpassed spike threshold, it was maintained by the voltage-gated, spike-generating conductances at a depolarized plateau upon which action potentials were fired. The potential of this plateau varied only slightly with spike frequency. We conclude that the voltage control exerted by the spike-generating currents in ganglion cells prevents a substantial response-dependent decrease in the electrical driving force of the excitatory currents, obviating the need for the voltage-independent synaptic efficacy provided by the combination of NMDA and non-NMDA inputs.

Effect of prenatal dexamethasone administration: fetal rabbit intestinal nutrient uptake and disaccharidase development

To examine the effect of prenatal steroids on fetal intestinal maturation, eight pregnant rabbits received either dexamethasone (Dex) or saline (Cont) on Days 25-27 of a 31-day gestation. As the rabbit provides a model of growth retardation based on uterine position, fetuses were identified as favored (Fav) or runt (Runt), generating four study groups: ContFav, ContRunt, DexFav, and DexRunt. On Day 31 the small intestinal uptake of glucose and proline was measured by an everted sleeve technique. Additionally, lactase and maltase activity was determined. Small intestinal length and nutrient uptake was significantly increased in the Dex fetuses. Control runts had a trend to decreased levels of nutrient uptake when compared to their favored counterparts. This trend reversed in the Dex fetuses with runt nutrient uptake surpassing that of the favored fetus. A trend to increased enzyme activity of both lactase and maltase was demonstrated. This report provides the first description of maternal steroid administration causing a marked increase in fetal small intestinal length and glucose and proline absorption in an in vivo model of intrauterine growth retardation.

Impairment of nasal mucociliary clearance during bone marrow transplantation. University of Nebraska Medical Center bone Marrow Transplantation Pulmonary Study Group

Transient mucositis occurs in almost all patients receiving bone marrow transplantation (BMT) but it is not known whether airway mucociliary transport is impaired during this period. We hypothesized that the preparative regimen associated with BMT impairs mucociliary clearance during the peri-transplant period. To test this hypothesis we determined nasal saccharine transit time (STT) and a mucositis score daily throughout the peri-transplant hospitalization in 13 patients receiving BMT. STT was prolonged in all patients at some time during the peri-transplant period and mucociliary clearance was ineffective in 12 of 13 patients for an average of 22% of the days. STT was most prolonged (about 155% of baseline) in the 2 week period following marrow infusion and correlated with the appearance of mucositis. STT returned to baseline values within 30 days after marrow infusion in all but one patient. These findings demonstrate that nasal mucociliary clearance is significantly impaired in most patients during BMT, if only temporarily. This decrease in local airway host defense may explain, in part, the increased risk of upper respiratory infections that these patients experience.

The contribution of NMDA and non-NMDA receptors to the light-evoked input-output characteristics of retinal ganglion cells

To examine how light-evoked excitatory synaptic inputs to retinal ganglion cells are transformed into output patterns of activity, action potentials were recorded with cell-attached patch-clamp techniques, and then EPSCs and EPSPs were recorded from the same cell in the whole-cell configuration. AP7, an NMDA antagonist, reduced the light-evoked peak spike frequency 36% +/- 21% (mean +/- SD) and reduced the EPSC amplitude, indicating a major role for NMDA receptors in the light response. CNQX, a non-NMDA receptor antagonist, reduced the light-evoked peak spike frequency 28% +/- 22%. CNQX also caused a voltage- and magnesium-dependent delay in spike onset. AP7 and CNQX, however, did not differ significantly in their effect on the EPSC time course, indicating that postsynaptic cellular properties are responsible for the delay observed in the presence of CNQX. These results show that the NMDA receptor contribution to the excitatory response is increased as the cell is depolarized from rest by non-NMDA input.