Wide field fluorescent imaging of extracellular spatiotemporal potassium dynamics in vivo

Potassium homeostasis is fundamental for the physiological functioning of the brain. Increased [K+] in the extracellular fluid has a major impact on neuronal physiology and can lead to ictal events. Compromised regulation of extracellular [K+] is involved in generation of seizures in animal models and potentially also in humans. For this reason, the investigation of K+ spatio-temporal dynamics is of fundamental importance for neuroscientists in the field of epilepsy and other related pathologies. To date, the majority of studies investigating changes in extracellular K+ have been conducted using a micropipette filled with a K+ sensitive solution. However, this approach presents a major limitation: the area of the measurement is circumscribed to the tip of the pipette and it is not possible to know the spatiotemporal distribution or origin of the focally measured K+ signal. Here we propose a novel approach, based on wide field fluorescence, to measure extracellular K+ dynamics in neural tissue. Recording the local field potential from the somatosensory cortex of the mouse, we compared responses obtained from a K+-sensitive microelectrode to the spatiotemporal increases in fluorescence of the fluorophore, Asante Potassium Green-2, in physiological conditions and during 4-AP induced ictal activity. We conclude that wide field imaging is a valuable and versatile tool to measure K+ dynamics over a large area of the cerebral cortex and is capable of capturing fast dynamics such as during ictal events. Moreover, the present technique is potentially adaptable to address questions regarding spatiotemporal dynamics of other ionic species. •Fluorescent ionic indicators were injected into the mouse cisterna magna.•Extracellular fluid was evenly stained allowing neocortical fluorescence imaging.•In vivo spatio-temporal K+ dynamics were imaged with wide field fluorescence.•Optical imaging correlated with K+ electrode measurements.•K+ concentration shifts were demonstrated in spontaneous/evoked epileptiform events.