2,3-Benzodiazepine derivatives are drug candidates synthesized for potential treatment of various neurodegenerative diseases involving the excessive activity of AMPA receptors. Here we describe a rapid kinetic investigation of the mechanism of inhibition of the GluR2Qflip AMPA receptor channel opening by two 2,3-benzodiazepine derivatives that are structurally similar (BDZ-2 and BDZ-3). Using a laser-pulse photolysis technique with a time resolution of approximately 60 mus, we measured the effects of these inhibitors on both the channel opening rate and the whole-cell current amplitude. We found that both compounds preferably inhibit the open-channel state, although BDZ-2 is a more potent inhibitor in that it inhibits the open-channel state approximately 5-fold stronger than BDZ-3 does. Both compounds bind to the same noncompetitive site. Binding of an inhibitor to the receptor involves the formation of a loose, partially conducting channel intermediate, which rapidly isomerizes to a tighter complex. The isomerization reaction is identified as the main step at which the receptor distinguishes the structural difference between the two compounds. These results suggest that addition of a bulky group at the N-3 position on the diazepine ring, as in BDZ-3, does not alter the mechanism of action, or the site of binding, but does lower the inhibitory potency, possibly due to an unfavorable interaction of a bulky group at the N-3 position with the receptor site. The new mechanistic revelation about the structure-reactivity relationship is useful in designing conformation-specific, more potent noncompetitive inhibitors for the GluR2 AMPA receptor.

2,3-Benzodiazepine derivatives are synthesized as drug candidates for a potential treatment of various neurodegenerative diseases involving the excessive activity of AMPA receptors. Here, we describe a rapid kinetic investigation of the mechanism of inhibition of the GluA2Qflip AMPA receptor channel opening by two 2,3-benzodiazepine derivatives, i.e. the prototypic 2,3-benzodiazepine compound GYKI 52466 [(1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine)] and 1-(4-aminophenyl)-3,5-dihydro-7,8-methylenedioxy-4H-2,3-benzodiazepin-4-one (BDZ-2). GYKI 52466 and BDZ-2 are structurally similar in that the 4-methyl group in the diazepine ring of GYKI 52466 is replaced by a carbonyl group, yielding BDZ-2. Using a laser-pulse photolysis technique with ∼60 μs time resolution, we characterize the effect of the two compounds individually on the channel-opening process of the GluA2Qflip receptor expressed in HEK-293 cells. We find that BDZ-2 preferentially inhibits the open-channel state, whereas GYKI 52466 is more selective for the closed-channel state of the GluA2Qflip receptors. Each inhibitor binds independently to its own noncompetitive site, yet the two sites do not interact allosterically. The significance of these results in the context of both the structure-activity relationship and the properties of the GluA2Qflip receptor channels is presented.

AMPA glutamate ion channels are tetrameric receptors in which activation to form the open channel depends on the binding of possibly multiple glutamate molecules. However, it is unclear whether AMPA receptors bound with a different number of glutamate molecules (i.e. one being the minimal and four being the maximal number of glutamate molecules) open the channels with different kinetic constants. Using a laser pulse photolysis technique that provides microsecond time resolution, we investigated the channel-opening kinetic mechanism of a nondesensitizing AMPA receptor, i.e. GluR1Q flip L497Y or a leucine-to-tyrosine substitution mutant, in the entire range of glutamate concentrations to ensure receptor saturation. We found that the minimal number of glutamate molecules required to bind to the receptor and to open the channel is two (or n ‫؍‬ 2), and that the entire channel-opening kinetics can be adequately described by just one channel-opening rate constant, k op , which correlates to n ‫؍‬ 2. This result suggests that higher receptor occupancy (n ‫؍‬ The ␣-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) 3 glutamate receptors are ligand-gated ion channels that are activated by binding of neurotransmitter glutamate (1, 2). An AMPA receptor is a tetrameric assembly with each subunit containing a glutamate binding site. The receptor can adopt multiple conductance levels, especially at high receptor occupancy, as observed in the single-channel records of wild-type and mutant recombinant receptors (3, 4) as well as native AMPA receptors (5). However, it remains unclear whether receptor occupancy plays a significant role in determining the kinetic constants for an ensemble rate process of channel opening as a function of glutamate concentration. The ensemble rate process is manifested in a whole-cell current response to the binding of glutamate in vitro and best represents the glutamatergic synaptic activity in vivo, such as excitatory postsynaptic current. Therefore, determining the number of glutamate molecules bound to a receptor or the percentage of the receptor occupancy pertinent to the rate of the channel opening is a basic question to be answered for understanding the function of AMPA receptors.To address this question, we investigated the channel-opening kinetics for a GluR1 AMPA receptor channel carrying a substitution of leucine (L) to tyrosine (Y) or L497Y. The discovery of this point mutation by Stern-Bach et al. (3) was a significant event in understanding the structure and function relationship of AMPA receptors in that (a) phenomenologically, the single leucine-to-tyrosine substitution renders the homomeric receptor channels virtually non-desensitizing (3), and (b) the phenotypic effect of this mutation is conserved at equivalent positions in all AMPA receptor subunits, i.e. GluR1-4 (6 -8). Furthermore, this mutation is thought to have no effect on either the main conductance level or the channel opening probability (3,7,9). From a crystallographic study, Sun et al. (6) revealed that this ...