Glycine at the NMDA receptor in cat visual cortex: saturation and changes with age.
1. Saturation of the glycine site at the N-methyl-D-aspartate (NMDA) receptor in cat visual cortex was tested by iontophoresing D-serine, and the contribution of this site to the visual response was tested by iontophoresing 7-chlorokynurenic acid (7-Cl-KYNA). Animals were tested at ages 3 wk, 6 wk, 5 mo, and adult. 2. In at least 40% of cells (24/57), D-serine increased the response significantly, showing that the glycine site was not saturated. However, the increase was rarely > 100%. 3. The amount by which D-serine increased the response did not vary significantly with age. 4. In five cases there was positive evidence for saturation of the glycine site. D-Serine did not increase the response significantly; however, when D-serine was applied on top of 7-Cl-KYNA, it did increase the response compared with that seen with 7-Cl-KYNA alone. These cases were all in 3-wk-old animals. In other cases (28/57), D-serine did not increase the response significantly, but we could not be absolutely certain that the D-serine had reached the site of action. 5. The effect of 7-Cl-KYNA was largest in 3-wk animals, in agreement with previous findings that the effect of D-2-amino-5-phosphonovalerate (APV) is largest at this age. 6. These results may provide a small part of the explanation for the finding that the NMDA receptor-mediated component of the visual response decreases between 3 and 6 wk of age, while the number of NMDA receptors is increasing. However, the magnitude of the results make it likely that other factors are more important.
Czepita
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Department of Ophthalmology and Visual Science, Yale University Medical School, New Haven, Connecticut 06520-8061, USA.
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D000375:Aging / Q000502:physiology*; D000818:Animals; D000831:Animals, Newborn; D002415:Cats; D005074:Evoked Potentials, Visual / Q000502:physiology; D005998:Glycine / Q000502:physiology*; D007478:Iontophoresis; D009474:Neurons / Q000502:physiology; D016194:Receptors, N-Methyl-D-Aspartate / Q000502:physiology*; D009435:Synaptic Transmission / Q000502:physiology*; D014793:Visual Cortex / Q000502:physiology*
D016194:Receptors, N-Methyl-D-Aspartate; D005998:Glycine