SOMATODENDRITIC CHLORIDE DYNAMICS ACROSS DIFFERENT CORTICAL NETWORK STATES

GABAergic signalling is classically considered to be inhibitory because the chloride influx through GABA_A receptors (GABA_ARs) hyperpolarises the postsynaptic membrane potential. However, when intracellular chloride concentrations ([Cl⁻]ᵢ) are elevated and the GABA_AR reversal potential shifts to more depolarised values, GABAergic signalling can become depolarising, which weakens synaptic inhibition and alters cortical synaptic integration.
Higher [Cl⁻]ᵢ levels are favoured during periods of elevated network activity, which are associated with increased presynaptic GABA release and depolarised postsynaptic membrane potentials. These factors increase the driving force for chloride to enter neurons, but their impact is predicted to vary across different neuronal compartments. Dendrites, for example, have small volumes and are therefore thought to be particularly susceptible to local changes in ion concentration. However, changes in chloride dynamics remain poorly understood under physiological conditions.
Here we develop and validate in vivo fluorescent reporters of somatodendritic chloride. We develop a Cre-dependent chloride sensor, mClY, and the first dendrite-targeted, all-optical reporter of chloride ion driving forces, which we call DT-ORCHID. We confirm the expression of both reporters along the somatodendritic axis of mouse cortical pyramidal neurons and then combine live cell imaging with simultaneous recordings of local field potentials. Consistent with biophysical predictions, intracellular chloride dynamics were found to differ along the somatodendritic axis as cortical networks oscillate between periods of intense neural activity (‘Up states’) and quiescence (‘Down states’). Together, this work provides a foundation for quantifying somatodendritic chloride dynamics across different physiological and pathological brain states in vivo.