The non-inactivating potassium M-current exerts a strong influence on neuronal excitability. The channels responsible for this current are made up of KCNQ subunits, and mutations in most of these produce human pathologies. Notably, in terms of excitation, mutations in either KCNQ2 or KCNQ3 lead to benign neonatal familial convulsions. Although a mere reduction of 25% in KCNQ2/3 function can increase excitability to epileptogenic levels, the potentiation of these subunits has anti-epileptogenic effects. After KCNQ2/3 heteromerization, current levels can augment as much as 10-fold, and we have discovered that there are three processes underlying this potentiation. First, there is an increase in the number of channels inserted in the membrane after heteromerization of the C-terminal region. Second, the N-terminal domain from KCNQ2 exerts a negative influence on the current level. Finally, Ala 315 of KCNQ3, a residue located in the inner vestibule after the selectivity filter, plays a critical role in preventing current flow in KCNQ3 homomeric channels, whereas it is permissive in heteromers in combination with Thr at the equivalent 276 position of KCNQ2.

In studying chimeras of NR2A and NR2C subunits of the NMDA receptor, we have found that glycine-independent desensitization depends on two regions of the extracellular N-terminal domain. One corresponds to a stretch of approximately 190 amino acids preceding the glutamate-binding domain S1. The other localizes at the interface between the N-terminal segment and the first transmembrane domain of NR2A subunits and involves A555 and S556. Both regions support desensitization in the absence of the other with different time courses. Desensitization did not develop with time in receptors containing the entire N-terminal region of NR2C. The introduction of A555 into the corresponding position of NR2C subunits enabled the receptors to manifest time-dependent increase in desensitization. Thus, this determinant behaves as an allosteric effector for glycine-independent desensitization.

1 This study was designed to assess the potential neuroprotective eect of several imidazol(ine) drugs and agmatine on glutamate-induced necrosis and on apoptosis induced by low extracellular K + in cultured cerebellar granule cells. 2 Exposure (30 min) of energy deprived cells to L-glutamate (1 ± 100 mM) caused a concentrationdependent neurotoxicity, as determined 24 h later by a decrease in the ability of the cells to metabolize 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazoliumbromide (MTT) into a reduced formazan product. L-glutamate-induced neurotoxicity (EC 50 =5 mM) was blocked by the speci®c NMDA receptor antagonist MK-801 (dizocilpine). 3 Imidazol(ine) drugs and agmatine fully prevented neurotoxicity induced by 20 mM (EC 100 ) Lglutamate with the rank order (EC 50 in mM): antazoline (13) 5 In HEK-293 cells transfected to express the NR1-1a and NR2C subunits of the NMDA receptor, antazoline and agmatine produced a voltage-and concentration-dependent block of glutamateinduced currents. Analysis of the voltage dependence of the block was consistent with the presence of a binding site for antazoline located within the NMDA channel pore with an IC 50 of 10 ± 12 mM at 0 mV. 6 It is concluded that imidazol(ine) drugs and agmatine are neuroprotective against glutamateinduced necrotic neuronal cell death in vitro and that this eect is mediated through NMDA receptor blockade by interacting with a site located within the NMDA channel pore.