The effects of copper ions on glutamate receptors in cultured rat cortical neurons

Copper plays an important role in the function of many physiological processes and can affect different neurotransmitter systems. In this study, we used the patch-clamp technique to investigate the effect of copper ions on glutamate receptors in cultured rat cortical neurons. CU2+ inhibited (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptors with an IC50 of 4.3 +/- 0.6 mu M (with 100 mu M kainate, holding potential -60 mV). The concentration-response could be best described by a two-site binding model. Moreover, copper reduced the efficacy of kainate at the AMPA receptor: in the presence of 30 mu M Cu2+, the EC(50) of kainate was shifted from 100.3 +/- 2.0 mu M to 329.9 +/- 31.4 mu M. The block by copper ions was not use-dependent. Complete recovery only occurred after the application of a high agonist concentration, or in the presence of the antioxidant dithiotreitol (DTT). A high concentration of histidine, a physiological ligand for Cu2+, did not augment the recovery. The kinetics of block were compared to those induced by 2,3-dihydro-6-nitro-7-sulfamoyl-benz(F)quinoxaline (NBQX), a well-described competitive antagonist of AMPA receptors. The onset, as well. as the offset of block by NBQX could be well approximated by single exponential functions with time constants of 0.28 +/- 0.02 and 0.87 +/- 0.09 s, respectively. Within seconds of wash-out of the antagonist, the response to kainate completely recovered. The kinetics of copper block were more complex: the block developed more slowly, and the onset, as well as the offset could be described by two exponential functions with quite different time constants (tau(on1), 0.8 +/- 0.13 s; tau(on2), 8.32 +/- 1.13 s; tau(off1), 0.17 +/- 0.01 s; tau(off2), 69 +/- 36.3 s). In addition to the described effects, Cu2+ also blocked currents induced by the application of N-methyl-D-aspartate (IC50: 15.0 +/- 2.6 mu M with 50 mu M NMDA). Based on these findings, a modulatory role of copper ions on the neurotransmission by excitatory amino acids is discussed.