Imaging synaptic inhibition in transgenic mice expressing the chloride indicator, Clomeleon

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Abstract
We describe here a molecular genetic approach for imaging synaptic inhibition. The thy-1 promoter was used to express high levels of Clomeleon, a ratiometric fluorescent indicator for chloride ions, in discrete populations of neurons in the brains of transgenic mice. Clomeleon was functional after chronic expression and provided non-invasive readouts of intracellular chloride concentration ([Cl−]i) in brain slices, allowing us to quantify age-dependent declines in resting [Cl−]i during neuronal development. Activation of hippocampal interneurons caused [Cl−]i to rise transiently in individual postsynaptic pyramidal neurons. [Cl−]i increased in direct proportion to the amount of inhibitory transmission, with peak changes as large as 4 mM. Integrating responses over populations of pyramidal neurons allowed sensitive detection of synaptic inhibition. Thus, Clomeleon imaging permits non-invasive, spatiotemporally resolved recordings of [Cl−]i in a large variety of neurons, opening up new opportunities for imaging synaptic inhibition and other forms of chloride signaling.