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 chloride-sensitive yellow fluorescent protein with the chloride-insensitive cyan fluorescent protein. The ratio of FRET-dependent emission of these fluorophores varied in proportion to the concentration of CI- and was used to measure intracellular chloride concentration ([CI-](i)) in cultured hippocampal neurons. [CI-](i) decreased during neuronal development, consistent with the shift from excitation to inhibition during maturation of GABAergic synapses. 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 dendrites, 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.