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 Cl? and was used to measure intracellular chloride concentration ([Cl?]i) in cultured hippocampal neurons. [Cl?]i decreased during neuronal development, consistent with the shift from excitation to inhibition during maturation of GABAergic synapses. Focal activation of GABAA receptors caused large changes in [Cl?]i that could underlie use−dependent depression of GABA−dependent synaptic transmission. GABA−induced changes in somatic [Cl?]i spread into dendrites, suggesting that [Cl?]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 Cl? signals in viv