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Uses of CNQX in Research

Excitatory synaptic transmission can be mediated through changing the responsiveness of AMPA (ɑ-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptors. One common method of altering responsiveness is changing the number of AMPA receptors in the postsynaptic membrane through endocytosis. Various stimuli, including CNQX, have diverse effects on AMPA receptor internalization. Known to be a competitive antagonist of the AMPA/kainate receptor, CNQX is used in studies investigating whether or not AMPA receptor endocytosis is ligand-dependent. In a culture of hippocampal neurons, CNQX partially inhibited AMPA receptor internalization that was stimulated by AMPA. However, when the hippocampal neurons were treated with CNQX alone, AMPA receptor internalization still took place in both the soma and dendrites. APV (NMDA receptor antagonist) or nimodipine (voltage gated calcium channel blocker) were also not able to block this internalization, suggesting that receptor activation is not a requirement for AMPA receptor endocytosis. The type of AMPA receptors endocytosed as a result of CNQX stimulation can also be identified using CNQX. In HEK cells tagged with GluR subunits, CNQX stimulates the internalization of GluR1 and GluR2 receptors. The intracellular region conserved in both GluR1 and GluR2 on the GluR2 C-terminal tail was identified and deleted. Deletion of this segment resulted in a decrease in constitutive endocytosis of the GluR2 receptor in HEK cells, pinpointing the sequence determining this effect.

CNQX is known to be a selective competitive antagonist for both AMPA receptors, which have an IC50 value of 400 nm, and kainate receptors, which have an IC50 value of 4 µM. It also noncompetitively inhibits NMDA receptors. CNQX is thus used to isolate GABAA receptor mediated spontaneous inhibitory postsynaptic currents. The actions of CNQX on the frequency of spontaneous inhibitory postsynaptic currents are independent of their actions at ionotropic glutamate receptors. Although the EC50 value of CNQX on the frequency of spontaneous inhibitory postsynaptic currents is similar to the IC50 value on kainate receptors, the blockade of kainate receptors is not responsible for the actions of CNQX. NBQX is a quinoxaline derivative that is known to be more effective than CNQX in blocking kainate receptors, but there was not a large increase in the frequency of spontaneous inhibitory postsynaptic currents. Additionally, CNQX’s effects were not replicated by kynurenate (glutamate receptor antagonist) or NS-102 (selective kainate receptor blocker) since there was no increase in the frequency of spontaneous inhibitory postsynaptic currents. Furthermore, D-AP5 and 7-CIK did not affect the frequency of spontaneous inhibitory postsynaptic currents, proving that the action of NMDA receptors do not account for the effects of CNQX.