Repetitive pH tests allow separation of true changes in the PM-inserted fraction of pHluorin-GluR-A receptors from insertion-unrelated fluctuations in the overall fluorescence. A, an example of glycine-induced changes in pHluorin-GluR-A fluorescence, which demonstrates that monitoring of changes in overall fluorescence may lead to underestimation of the actual increase in PM-inserted receptor fraction. In contrast, monitoring changes in the fast phase of fluorescence quenching allows a more accurate estimate of true changes in the PM-inserted fraction. The respective values are shown as bars in the right panel. B, an example from another neuron that was monitored under control conditions (i.e., without applying gly/0 Mg2+) showing that an increase in the overall fluorescence is not always indicative of the PM insertion of new receptors. The bars in the right panel demonstrate that the amplitude of the fast phase of fluorescence quenching remains unchanged in spite of an increase in baseline fluorescence.