[H]Spermine binding to synaptosomal membranes from the chick retina

The binding of ³H]spermine to synaptosomal membranes from chick retina was examined. Saturable specific binding of ³H]spermine to synaptosomal membranes from plexiform layers of retina (P₁ and P₂) has been characterized, and found to concentrate in the inner plexiform layer compared to the outer plexiform layer (B_max=9.3 and 37 pmol/mg protein for P₁ and P₂, respectively). Kinetics of specific ³H]spermine binding yield a sigmoidal saturation curve, indicating positive cooperativity (nH: 2.4 and 3.2 for P₁ and P₂, respectively) with high affinity: K_app=61 and 67 nM for P₁ and P₂. The time required to attain equilibrium at room temperature was less than 5 min in both fractions. Dose–response curves for spermine, spermidine, and diethylene–triamine (DET) show different potencies for inhibiting ³H]spermine binding: spermine>spermidine>DET. Our results support a role for polyamines (PA) as neurotransmitters or neuromodulators in the vertebrate retina.