Different specific binding sites of [3H]glycine and [3H]strychnine in synaptosomal membranes isolated from frog retina.

Synaptosomal fractions were isolated from frog retina: a fraction enriched in photoreceptor terminals (P1) and a second one (P2) containing interneurons terminals. We compared the binding of [3H]glycine and [3H]strychnine to membranes of these synaptosomes. The binding of both radioactive ligands was saturable and Na(+)-independent. [3H]Glycine bound to a single site in P1 and P2 synaptosomal fractions, with KD = 12 and 82 nM and BMax = 3.1 and 3.06 pmol/mg protein respectively. [3H]Strychnine bound to two sites in each one of the synaptosomal fractions. For P1 KD values were 3.9 and 18.7 nM, and BMax values were 1.1 and 7.1 pmol/mg protein, respectively. Membranes from the P2 synaptosomal fraction showed KD's of 0.6 and 48 nM and BMax's of 0.4 and 4.5 pmol/mg. Specific [3H]glycine binding was displaced by beta-alanine, 1-serine, d-serine and HA966, but not by strychnine, 7-chlorokynurenic or 5,7-dichloro-kynurenic acids. Specific [3H]strychnine binding was partially displaced by glycine and related amino acids and totally displaced only by 2-NH2-strychnine. Our results indicate the presence of high affinity binding sites for glycine and strychnine in frog retinal synaptosomal membranes. The pharmacological binding pattern indicates the presence of the strychnine sensitive glycine receptor as well as other sites. These might not include the NMDA receptor-associated glycine site.