Organic blockers escape from trapping in the AMPA receptor channels by leaking into the cytoplasm

The voltage-dependent block of AMPA receptor (AMPAR) channels by a series of dicationic compounds was studied on native GluR2-lacking receptors of striatal giant interneurons isolated from rat brain slices. The dicationic derivatives of adamantane, dimethyladamantane, diphenyl, and phenylcyclohexyl were used. Voltage dependence of the blockade and of the unblocking rate suggests that the compounds permeate the open AMPAR channels. The permeation of adamantane derivatives was demonstrated previously. However, for derivatives of phenylcyclohexyl this finding is surprising because of the large dimensions of the phenylcyclohexyl moiety. All these compounds were found to get trapped in the closed state of the channel. However, time-dependent decrease of trapping was found. This effect is accelerated by hyperpolarization, suggesting that blockers can escape from trapping into the cytoplasm. Importantly, there is a correlation between permeation through the open channel and escape from trapping. Dicationic compounds were shown to block open and closed AMPAR channels from the inside of the cell. Thus, trapping of AMPAR channel blockers after agonist removal does not prevent escape of blockers into the cytoplasm. It is concluded that closure of the AMPAR channel gates at the extracellular vestibule is not coupled with plugging of the pathway between the selectivity filter and cytoplasm. Possible physiological importance of this blocking mechanism is discussed.