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) r eceptor in cat visual cortex was tested by iontophoresing D-serine, an d the contribution of this site to the visual response was tested by i ontophoresing 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 (24/5 7), D-serine increased the response significantly, showing that the gl ycine site was not saturated. However, the increase was rarely > 100%. 3. The amount by which D-serine increased the response did not vary s ignificantly with age. 4. In five cases there was positive evidence fo r saturation of the glycine site. D-Serine did not increase the respon se significantly; however, when D-serine was applied on top of 7-Cl-KY NA, it did increase the response compared with that seen with 7-Cl-KYN A 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 coul d 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 agr eement with previous findings that the effect of D-2-amino-5-phosphono valerate (APV) is largest at this age. 6. These results may provide a small part of the explanation for the finding that the NMDA receptor-m ediated 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 i mportant.