Understanding The Single Channel Behavior Of Glua3 Receptors

A tremendous amount of research has been dedicated to elucidating the functional mechanism of glutamate receptors. AMPA receptors in particular were the first in which structure was correlated to function; specifically, the structures of the isolated ligand binding domain (LBD) bound to different agonists, partial agonists, and antagonists, were compared to whole cell electrophysiological recordings. A correlation between the degree of lobe closure and maximal currents was discovered; that is, the greater the degree of LBD closure, the larger the measured currents. However, as more information relating LBD closure to channel activation began to surface, it became clear the relationship was not as direct as previously thought. The studies in this dissertation look at the single channel behavior of GluA3 receptors and supply new evidence that both full and partial agonists induce a similar mechanism of activation on AMPA receptors. These studies examine how subtle changes in the LBD can affect single channel properties. GluA3 channels have three conductance levels and the probability of opening to any of these levels is dependent on the agonist; that is, full agonists are more likely to open the channel to the largest conductance level than partial agonists. GluA3 channels also display modal behaviors that are determined by the open probability and vary from very low to very high. A never before seen fast channel blocking effect is also reported. These modal behaviors are present at low agonist concentrations and observed for all the agonists tested (glutamate, FW, ClW and NO2W). The agonists tested were all able to induce full LBD closures. The stability of the closures differed in the D655/S656 peptide conformation and the M712 orientation. D655/S656 flipped conformation allows H-bonding across the lobes to occur, stabilizing LBD closures. Bulkier substituents in the 5 position of the willardiine compounds reorients M712, which could alter the M4 transmembrane helix, thereby affecting channel gating. Based on these studies, although the mechanism of activation is similar for the agonists tested, it seems that subtle dynamic conformations of amino acids in the LBD caused by agonist binding can affect the agonist induced activation