Information processing in the vertebrate retina occurs in two separate channels known as ON and OFF channels. When intracellular electrophysiological recordings were obtained from the perfused retina-eyecup preparation of the mud-puppy (Necturus maculosus), the addition of 2-amino-4-phosphonobutyric acid to the bathing medium blocked all responses in the ON channel but left intact the OFF responses including OFF ganglion cell discharge. 2-Amino-4-phosphonobutyric acid blocks the light response of the ON bipolar cell by mimicking the endogenous photoreceptor transmitter.

A lthough bacteria and many invertebrate species use both Dand L-enantiomers of amino acids for cellular functions, it was generally believed that higher organisms had a more restricted stereospecificity and were confined to the use of L-amino acids. Thus, the D-amino acids detected in vertebrates were generally assumed to come from ingested material or intestinal flora (1). Indeed, the enzyme D-amino acid oxidase (D-AAO), which degrades many D-amino acids, is found in vertebrate tissues, but it was assumed to exist for degrading D-amino acids from external sources. However, in the past decade, it has become clear that some D-amino (6) proposed that D-serine released by glia plays a regulatory role as a necessary coagonist for the glutamate activation of NMDA receptors. Thus, an expanded model for glutamate synapses emerged in which perisynaptic glial processes respond to glutamate released from the presynaptic neuron by releasing D-serine, which, in turn, facilitates the activation of NMDA receptors on the postsynaptic cell. The functional importance of D-serine was further supported by the more recent discovery that the enzyme serine racemase, which converts L-serine to D-serine, is localized to astrocytes in the nervous system (7). The discovery of this mechanism adds to the accumulating evidence that glial cells dynamically influence neuronal activity.Although D-serine and serine racemase have been found in many regions of the brain, no study has yet been carried out in the vertebrate retina, a tissue that offers several advantages for studying the actions of D-serine. The retinal network is a well-characterized region of the central nervous system that can be studied in an intact preparation, permitting the use of light stimulation to provide natural activation of the neural circuitry and test more directly the physiological significance of D-serine. Virtually all retinal ganglion cells have AMPA and NMDA receptors, which both contribute to light-evoked excitation (8-12), and NMDA receptor currents can be enhanced through pharmacological techniques and studied in relative isolation from AMPA receptor contributions (13).In the present study, we have used immunohistochemistry, immunoblotting, analytical chemistry (HPLC), and electrophysiology to evaluate the presence of D-serine and serine racemase in the retinas of several vertebrate species. Our findings indicate that D-serine and serine racemase are present in the retina and seem to be localized to Müller glial cells and astrocytes. In addition, we have demonstrated that exogenous D-serine can enhance NMDA receptor responses and modulate light-evoked activity in retinal ganglion cells. The enhancement of NMDA receptor function seems to involve both tonic and phasic components of glutamatergic input. When the D-serine-degrading enzyme D-AAO (14) is added to the bathing medium, currents mediated by NMDA receptors are reduced, suggesting that these responses depend on endogenous D-serine as a coagonist for the glutamate activation of NMDA receptors. Method...

We recently showed that a rare cell from murine bone marrow, which we termed multipotent adult progenitor cells (