Specific mechanism of use-dependent channel block of calcium-permeable AMPA receptors provides activity-dependent inhibition of glutamatergic neurotransmission

Non-technical summary Calcium-permeable (CP) AMPA receptors play an important role in many synaptic functions and neuronal death. In this study, we investigated the blocking activity of a selective CP-AMPAR channel blocker, IEM-1925, and discovered that the blocker preferentially inhibits receptor activity at an increased frequency of synapse stimulation and in the continuous presence of the agonist. The activity-dependent block described in this study may serve as a useful means for answering important questions about the specific role of CP-AMPARs, as it opens a new way to modulate CP-AMPAR-mediated transmission using a physiologically relevant approach. It allows for the examination of specific involvement of CP-AMPARs in the physiological and pathological processes, including high-frequency synaptic activity or increase of the steady-state glutamate concentration.This study examined the blocking action of the selective channel blocker of calcium-permeable (CP) AMPA receptors, N1-(1-phenylcyclohexyl)pentane-1,5-diaminium bromide (IEM-1925), on excitatory postsynaptic currents in rat neostriatal and cortical neurons and in fly neuromuscular junctions. In both preparations, the blocking of CP-AMPA receptor currents increased along with the stimulation frequency. The continuous presence of kainate, which activates AMPA receptors, in the external solution also caused an enhanced blocking effect. Likewise, decrease of the synaptic release by lowering calcium concentration resulted in significant reduction of the blocking action. The activity dependence of the block is explained using the guarded receptor model. The drug molecule can only bind if the channel is open. After the channel has closed, the drug molecule remains trapped inside. However, the trapped molecule slowly egresses from closed channels to the cytoplasm. The total block effect is determined by the equilibrium between accumulation of the drug in the open channels and relief from the closed channels. Therefore, the conditions that favour the open state result in enhanced inhibition. This significant finding reveals a new way to modulate CP-AMPAR-mediated transmission using a physiologically relevant approach. Moreover, it allows the involvement of CP-AMPARs in the physiological and pathological processes - such as high-frequency synaptic activity or increase of the steady-state glutamate concentration - to be examined.