Spermine inhibits [3H]glycine binding at the NMDA receptors from plexiform layers of chick retina.

Saturable specific binding of glycine to synaptosomal membranes from plexiform layers of the retina has been described, which seems to correspond to the modulatory site on NMDA-receptors (26). Spermine inhibited specific [3H]glycine binding to membranes from synaptosomal fractions from the outer (P1) and the inner (P2) plexiform layers of 1-3 day-old chick retinas in a dose-dependent manner with an IC50 = 35 microM for the P1 fraction and 32 microM for the P2 fraction. Kinetic experiments and non-linear regression analysis of [3H]glycine-specific binding showed a Kd approximately 100-150 nM in both fractions, and a higher Bmax (4.11 +/- 0.47 pmol/mg protein) for the inner plexiform layer compared to the outer plexiform layer (Bmax = 2.76 +/- 0.25 pmol/mg protein). Strychnine-insensitive [3H]glycine binding was inhibited by 100 microM spermine, due to a reduction in Bmax (P1 = 0.84 +/- 0.16 pmol/mg protein; P2 = 0.81 +/- 0.16 pmol/mg protein) without affecting the Kd. Association and dissociation constants in the absence and presence of 50 microM spermine remained unchanged. Results demonstrate the presence of a single modulatory site for spermine on NMDA receptors, in both synaptic layers of the chick retina.