INHIBITION OF AMPA RECEPTOR-STIMULATED CO-57(2-2-AMINO-4-PHOSPHONOBUTANOIC-ACID AND L-2-AMINO-4-PHOSPHONOBUTANOIC-ACID (D-AP4 AND L-AP4) AND L-SERINE-O-PHOSPHATE (L-SOP) IN CULTURED CEREBELLAR GRANULE CELLS() INFLUX BY D)

This study describes the inhibition of Co-57(2+) influx through Ca2+-p ermeable lpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA ) receptors, consequent to the application of L-2-amino-4-phosphonobut anoic acid (L-AP4), D-AP4 and L-serine-O-phosphate (L-SOP) in cultured cerebellar granule cells. The forskolin-stimulated accumulation of cy clic AMP was inhibited by (2S,1'S,2'S)-2-(carboxycyclopropyl)glycine ( L-CCG-1) with an IC50 = 491 +/- 135 nM and by L-AP4 in a biphasic mann er (IC50(1) = 232 +/- 61 nM and IC50(2) = > 300 mu M), confirming the presence of group II and group III mGlu receptors, respectively. Co-57 (2+) influx was stimulated by kainate (EC50 = 42.2 +/- 11.3 mu M) and, in the presence of 30 mu M cyclothiazide, by (S)-5-fluorowillardiine (EC50 = 0.7 +/- 0.1 mu M) and (S)-AMPA (EC50 = 2.8 +/- 0.5 mu M). The effects of the latter were abolished by 10 mu M 6-nitro-7-sulphamoylbe nzo[f]quinoxaline-2,3-dione (NBQX). L-AP4 (IC50 = > 300 mu M) D-AP4 (I C50 = > 100 mu M) and L-SOP (IC50 = 199 +/- 6 mu M) inhibited 6 mu M ( S)-AMPA-stimulated Co-57(2+) influx, whereas L-CCG-1 (up to 10 mu M), 300 mu M (RS)-3,5-dihydroxyphenylglycine, 300 mu M (+/-)-baclofen and 1 mM carbachol were ineffective. Pre-incubation with either pertussis toxin (250 ng/ml, 48 hr), 1 mM dibutyryl cyclic AMP, or the potent gro up III mGlu receptor antagonist (RS)-alpha-cyclopropyl-4-phosphonophen ylglycine ((RS)-CPPG), tested at 400 mu M, failed to alter the inhibit ion of AMPA receptor activity by 300 mu M L-SOP. Unlike 10 mu M NBQX, neither L-AP4, D-AP4 or L-SOP (tested at 1 mM) inhibited the binding o f 10 nM (S)-[H-3]S-fluorowillardiine (a selective AMPA receptor ligand ) to granule cell membranes. Therefore, in these neurones, high concen trations (> 100 mu M) of L-AP4, L-SOP and D-AP4 inhibit Ca2+-permeable AMPA receptors by a mechanism distinct from known mGlu receptor actio n and at a site independent from that for AMPA receptor agonists. (C) 1997 Elsevier Science Ltd.