Figure 1. Frequency-dependent facilitation of currents through Ca²⁺-permeable AMPA receptors.

In the closed state the inner pores of glutamate receptors are tonically blocked by polyamines (left). Following receptor activation only a modest amount of current flows due to the occlusion of the pore by polyamines. On repetitive activation significant relief of channel block occurs allowing greater current flow through the channel. Consequently, at a given voltage, current amplitudes are maximal only once the significant polyamine block is removed (right).