AP4 inhibits chloride-dependent binding and uptake of [3H]glutamate in rabbit retina

Glutamate is one of the major neurotransmitters used by primary and secondary neurons of the visual pathway in retina. AP4(2-amino-4-phosphonobutyric acid) preferentially blocks the activity of one functional subclass of retinal neurons, ON bipolar cells, apparently by acting as an agonist at a hyperpolarizing glutamate receptor. We have used in vitro binding assays to examine different subclasses of presumptive glutamate receptors in retinal membrane fractions. One subclass consists of AP4-sensitive binding sites which require calcium and chloride for maximal binding and which are inhibited by freeze-thaw procedures. In addition, AP4 inhibits chloride-dependent [3H]glutamate uptake into retinal synaptosomes and intact retina. [3H]glutamate which is accumulated via the AP4-sensitive mechanism can be subsequently released by depolarizing levels of potassium. The pharmacological selectivity of AP4-sensitive glutamate receptors on ON bipolar cells measured electrophysiologically is very similar to that of AP4-sensitive, [3H]glutamate binding and uptake, measured biochemically in subcellular fractions. These results raise the possibility that AP4-sensitive glutamate recognition sites in retina may be linked to two separate effectors, one which gates ion channels and leads to hyperpolarization, and another which acts as a glutamate transporter.