A genetically encoded ratiometric indicator for chloride: Capturing chloride transients in cultured hippocampal neurons

We constructed a novel optical indicator for chloride ions by fusing the ch loride-sensitive yellow fluorescent protein with the chloride-insensitive c yan fluorescent protein. The ratio of FRET-dependent emission of these fluo rophores varied in proportion to the concentration of CI- and was used to m easure intracellular chloride concentration ([CI-](i)) in cultured hippocam pal neurons. [CI-](i) decreased during neuronal development, consistent wit h the shift from excitation to inhibition during maturation of GABAergic sy napses. Focal activation of GABA(A) receptors caused large changes in [CI-] (i) that could underlie use-dependent depression of GABA-dependent synaptic transmission. GABA-induced changes in somatic [CI-](i) spread into dendrit es, suggesting that [CI-](i) can signal the location of synaptic activity. This genetically encoded indicator will permit new approaches ranging from high-throughput drug screening to direct recordings of synaptic CI- signals in vivo.