Eric R. Stevens

d-serine and serine racemase are present in the vertebrate retina and contribute to the physiological activation of NMDA receptors

d-serine has been proposed as an endogenous modulator of N-methyl-d-aspartate (NMDA) receptors in many brain regions, but its presence and function in the vertebrate retina have not been characterized. We have detected d-serine and its synthesizing enzyme, serine racemase, in the retinas of several vertebrate species, including salamanders, rats, and mice and have localized both constituents to Müller cells and astrocytes, the two major glial cell types in the retina. Physiological studies in rats and salamanders demonstrated that, in retinal ganglion cells, d-serine can enhance excitatory currents elicited by the application of NMDA, as well as the NMDA receptor component of light-evoked synaptic responses. Application of d-amino acid oxidase, which degrades d-serine, reduced the magnitude of NMDA receptor-mediated currents, raising the possibility that endogenous d-serine serves as a ligand for setting the sensitivity of NMDA receptors under physiological conditions. These observations raise exciting new questions about the role of glial cells in regulating the excitability of neurons through release of d-serine.

Effect of ondansetron, a 5-HT3 receptor antagonist, on the dynamic association between bulimic behaviors and pain thresholds.

Abstract Thresholds for detection of both pressure and thermal pain are elevated in patients with bulimia nervosa. The present study was aimed at determining (1) if pressure pain detection thresholds (PDT) varied dynamically with the primary disease symptoms of binge eating and vomiting and (2) if the elevation in PDT was effected by treatment with ondansetron (ONDAN), a 5‐HT3 receptor antagonist. PDT was defined as the mean of the minimal amount of pressure (measured in g) perceived as painful when exerted by a 1 mm2 blunted point onto the center of the ventral surface of the ungual phalanx of digits 2‐5 of the non‐dominant hand. Fourteen female patients with severe bulimia nervosa (currently >seven binge/vomit episodes per week;>2 years illness duration) served as participants. PDT were evaluated at weekly intervals during the course of ongoing treatment studies (double‐blind and ‘open’ label) investigating the therapeutic effects of ONDAN. Data were analyzed by random regression analyses, allowing for the repeated‐measures and non‐orthogonal design. Data collected from 14 patients under the no‐drug condition indicated that PDT increased over the interval between binge/vomit episodes, with significant elevations occurring at times when patients had naturally exceeded their average inter‐binge interval. Eleven of these 14 patients underwent 4 weeks of ONDAN treatment. Under this drug condition, the time since the last binge/vomit episode was no longer a significant predictor of PDT. These patients also experienced a significant reduction in the frequency of bulimic behaviors, a finding reported in detail elsewhere. The above finding from untreated patients support the involvement of a common underlying mechanism driving both the increase in pain detection thresholds and the occurrence of the next bulimic episode. This possibility is further supported by the findings that ONDAN treatment is associated with a significant moderation of both variables. The effect of ONDAN may be mediated by blockade of afferent vagal neurotransmission, although other mechanisms must be considered.

Glycine transport accounts for the differential role of glycine vs. d‐serine at NMDA receptor coagonist sites in the salamander retina

In this study, we demonstrate that d‐serine interacts with N‐methyl‐d‐aspartate receptor (NMDAR) coagonist sites of retinal ganglion cells of the tiger salamander retina by showing that exogenous d‐serine overcomes the competitive antagonism of 7‐chlorokynurenic acid for this site. Additionally, we show that exogenous d‐serine was more than 30 times as effective at potentiating NMDAR currents compared with glycine. We thus examined the importance of glycine transport through the application of selective antagonists of the GlyT1 (NFPS) and GlyT2 (ALX‐5670) transport systems, while simultaneously evaluating the degree of occupancy of the NMDAR coagonist binding sites. Analysis was carried out with electrophysiological recordings from the inner retina, including whole‐cell recordings from retinal ganglion cells and extracellular recordings of the proximal negative field potential. Blocking the GlyT2 transport system had no effect on the light‐evoked NMDAR currents or on the sensitivity of these currents to exogenous d‐serine. In contrast, when the GlyT1 system was blocked, the coagonist sites of NMDARs showed full occupancy. These findings clearly establish the importance of the GlyT1 transporter as an essential component for maintaining the coagonist sites of NMDARs in a non‐saturated state. The normal, unsaturated state of the NMDAR coagonist binding sites allows modulation of the NMDAR currents, by release of either d‐serine or glycine. These results are discussed in light of contemporary findings which favor d‐serine over glycine as the major coagonist of the NMDARs found in ganglion cells of the tiger salamander retina.

Light-evoked NMDA receptor-mediated currents are reduced by blocking D-serine synthesis in the salamander retina

Experiments were carried out in the retina of the tiger salamander (Ambystoma tigrinum) to evaluate the importance of D-serine synthesis on light-evoked N-methyl D-aspartate (NMDA) receptor-mediated components of ganglion cells and contributions to the proximal negative field potential. We blocked the synthesis of D-serine through brief exposures of the retina to phenazine ethosulfate and validated the changes in the tissue levels of D-serine using capillary electrophoresis methods to separate and measure the amino acid enantiomers. Ten minute exposures to phenazine ethosulfate decreased D-serine levels in the retina by about 50% and significantly reduced the NMDA receptor contribution to light responses of the inner retina. This is the first report of a linkage between D-serine synthesis and NMDA receptor activity in the vertebrate retina.