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 [3H]-(2S,4R)-4-methylglutamate to rabbit whole-brain membranes. [3H]-(2S,4R)-4-methylglutamate binding was rapid, reversible and labelled two sites (KD1 = 3.67+/-0.50 nM/Bmax1 = 0.54+/-0.03 pmol/mg protein and KD2 = 281.66+/-12.33 nM/ Bmax2 = 1.77+/-0.09 pmol/mg protein). [3H]-(2S,4R)-4-methylglutamate binding was displaced by several non-NMDA receptor ligands: domoate > kainate >> L-quisqualate > or = L-glutamate > 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) >> (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 microM. We conclude that [3H]-(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.
Toms
N J
NJ
Department of Pharmacology, School of Medical Sciences, University of Bristol, UK.
Reid
M E
ME
Phillips
W
W
Kemp
M C
MC
Roberts
P J
PJ
D000818:Animals; D001667:Binding, Competitive / Q000187:drug effects; D001923:Brain Chemistry / Q000187:drug effects*; D005971:Glutamates / Q000138:chemical synthesis / Q000378:metabolism*; D066298:In Vitro Techniques; D007700:Kinetics; D008024:Ligands; D011817:Rabbits; D011869:Radioligand Assay; D018091:Receptors, AMPA / Q000378:metabolism; D018092:Kainic Acid Receptors / Q000187:drug effects*; D013570:Synaptic Membranes / Q000187:drug effects / Q000378:metabolism*
D005971:Glutamates; D008024:Ligands; D018091:Receptors, AMPA; D018092:Kainic Acid Receptors; C046105:4-methylglutamic acid