Characterization of quisqualate-type L-glutamate receptors in the retina.
In the vertebrate retina excitatory transmission seems to be mediated mainly by excitatory amino acids; glutamate and/or aspartate are the most viable candidates to subserve this function. Postsynaptic receptors for N-methyl-D-aspartate (NMDA), kainate (KA), quisqualate (QA) and 2-amino-4-phosphonobutyric acid have been electrophysiologically identified. In this work we have tried to identify and characterize QA receptors through the binding of the most specific analogue available for this receptor: [3H]alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid ([3H]AMPA). AMPA binding to retinal membranes was sodium- and temperature-independent, with optimum pH at 6-7. Ligand-receptor interaction was reversible and saturable. Pharmacologically, glutamate analogues were more active displacers than NMDA analogues: AMPA greater than (RS)-3-hydroxy-4,5,6,7-tetrahydro-isoxazolo-(5,4-C)-pyridine-7-car boxylic acid = L-Glu = QA; with IC50 in the low microM range. Glutamic acid diethylester was uneffective while KA and cis-2,3-piperidine dicarboxylate were potent inhibitors of binding. Binding was stereospecific, L-isomers being more effective displacers than D-forms. Subcellular distribution showed binding concentrated in the inner plexiform layer (IPL), but also present in the outer plexiform layer (OPL). Kinetics of [3H]AMPA binding showed a high affinity kB = 1-2 microM in membranes from complete retina, IPL and OPL, with binding sites concentrated in P2 (Bmax = 16.2 pmol/mg protein). Our results provide biochemical evidence for the presence and distribution of physiologically relevant QA receptors in the chick retina which is in agreement with previous physiological findings.
D000818:Animals; D000831:Animals, Newborn; D001224:Aspartic Acid / Q000378:metabolism; D001667:Binding, Competitive; D002645:Chickens; D005971:Glutamates / Q000378:metabolism; D018698:Glutamic Acid; D006863:Hydrogen-Ion Concentration; D007051:Ibotenic Acid / Q000031:analogs & derivatives / Q000378:metabolism; D007700:Kinetics; D018091:Receptors, AMPA; D011955:Receptors, Drug / Q000032:analysis* / Q000378:metabolism; D017470:Receptors, Glutamate; D017981:Receptors, Neurotransmitter / Q000032:analysis*; D012160:Retina / Q000032:analysis*; D013347:Subcellular Fractions / Q000378:metabolism; D018350:alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid
D005971:Glutamates; D018091:Receptors, AMPA; D011955:Receptors, Drug; D017470:Receptors, Glutamate; D017981:Receptors, Neurotransmitter; D007051:Ibotenic Acid; D001224:Aspartic Acid; D018698:Glutamic Acid; D018350:alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid