Abstract
Abstract: :
Purpose: The purpose of this study was to determine whether D–serine or glycine acts as the endogenous ligand at the glycine binding site of NMDA receptors on ganglion cells in the vertebrate retina, and to elucidate the mechanisms involved in the selection of one amino acid over the other. Methods: Whole–cell recordings from ganglion cells in a tiger salamander (Ambystoma tigrinum) eyecup preparation revealed NMDA receptor mediated currents in response to light. Ganglion cells were identified by labelling the cut optic nerve fibers with tetramethylrhodameine. A gravity fed perfusion system delivered Ringer's solution and drugs to the tissue at a rate of 1–2 ml/min. Light stimuli were formed by focusing the image of an LCD panel through a water immersion objective onto the plane of the retina. Results: Exogenously applied D–serine potentiated NMDA receptor mediated responses at lower concentrations than glycine. Blocking the high–affinity glycine transporter GlyT1 with N[3–(4–fluorophenyl)–3–(4–phenylphenoxy)–propyl]sarcosine (NFPS) resulted in saturation of the NMDA receptor glycine binding site. Inhibition of D–serine synthesis with the irreversible serine racemase inhibitors, methyl– or ethyl–phenazine, significantly suppressed the NMDA receptor mediated component of the light response. This NMDA receptor inhibition was overcome with the addition of exogenous D–serine or NFPS. Conclusions: These results suggest that D–serine synthesized in retial glial cells acts as the preferred endogenous ligand for the NMDA receptor glycine binding site in spite of the fact that glycine and D–serine are both present in the vertebrate retina. Glycine is apparently prevented from interacting with retinal NMDA receptors by the high–affinity glycine transporter GlyT1, whereas D–serine appears to have relatively unrestricted access to these receptors.
Keywords: Muller cells • receptors: pharmacology/physiology • neurotransmitters/neurotransmitter systems