Two-photon chloride imaging in neurons of brain slices

Two-photon laser scanning microscopy has been used successfully for imaging activity-dependent changes of intracellular calcium and sodium levels. Here we introduce a simple technique for two-photon chloride imaging in intact neurons. It involves the use of the membrane-permeable Cl- indicator dye MQAE [N-(6-methoxyquinolyl) acetoethyl ester]. Brief incubation with MQAE produced a robust loading of cells in slices from various brain regions including hippocampus, cortex and cerebellum. In contrast to conventional fluorescence measurements using MQAE, two-photon imaging was not affected in a major way by dye bleaching and phototoxic damage. Instead, it allowed prolonged recordings of time-resolved changes in intracellular chloride concentration in somata and dendrites. As an example of an application we imaged GABA-mediated Cl- transients in pyramidal cells of cortical and hippocampal slices as well as in cerebellar Purkinje neurons. By combining Cl- imaging with the gramicidin-based perforated-patch-clamp technique we showed that changes in MQAE fluorescence are proportional to the magnitudes of GABA-evoked transmembrane Cl- fluxes. Thus, MQAE-based two-photon microscopy promises to be a valuable technique for many applications requiring chloride imaging in single cells.