A novel kainate receptor ligand [3H]-(2S,4R)-4-methylglutamate:: Pharmacological characterization in rabbit brain membranes

Since kainate evokes large non-desensitizing currents at alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, kainate is of limited use in discriminating between AMPA and kainate receptors. Following recent reports that (2S,4R)-4-methylglutamate is a kainate receptor-selective agonist, we have radiolabelled and subsequently characterized the binding of [H-3]-(2S,4R)-4-methylglutamate to rabbit whole-brain membranes. [H-3]-(2S,4R)-4-methylglutamate binding was rapid, reversible and labelled two sites (K-D1 = 3.67 +/- 0.50 nM/B-max1 = 0.54 +/- 0.03 pmol/mg protein and K-D2 = 281.66 +/- 12.33 nM/ B-max2 = 1.77 +/- 0.09 pmol/mg protein). [H-3]-(2S,4R)-4-methylglutamate binding was displaced by several non-NMDA receptor ligands: domoate > kainate much greater than L-quisqualate greater than or equal to L-glutamate > 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) much greater than (S)-AMPA = (S)-5-fluorowillardiine > NMDA. Neither the metabotropic glutamate receptor agonists (1S,3R)-ACPD or L-AP4, together with the L-glutamate uptake inhibitor L-trans-2,4-PDC, influenced binding when tested at 100 mu M. We conclude that [H-3]-(2S,4R)-4-methylglutamate is a useful radioligand for labelling kainate receptors. It possesses high selectivity, and possesses a pharmacology similar to that for rat cloned low-affinity (Glu5 and 6) kainate receptor subunits. (C) 1998 Elsevier Science Ltd. All rights reserved.