ETD

• calcium
• chloride
• excitation
• GCaMP
• iClima
• imaging
• inhibition
• neuroscience

The interplay between excitation and inhibition has always been a fundamental question regarding the functioning of the brain; in cortical circuits, excitatory and inhibitory signals dynamically interact to shape neuronal responses and to enable precise computations. Despite its importance, our understanding of how excitation and inhibition are coordinated at the population level in vivo remains limited, largely due to the technical challenges associated with measuring both processes simultaneously with high spatial and temporal resolution.
Previous work in our lab has exploited a novel genetically encoded ratiometric sensor, called iClima (improved Cl- imaging), and has demonstrated its ability to detect intracellular chloride concentration changes in both physiological and pathological conditions. In the present study, we co-expressed a calcium indicator (GCaMP6f) and a chloride sensor (iClima) in mice V1 cortical neurons, and achieved simultaneous optical readout of excitatory and inhibitory signals within the same neuronal populations. We used in vivo two-photon imaging on anesthetized, head-fixed mice, and acquired responses during both spontaneous and visually evoked activity; we recorded the activity in response to different visual stimuli, mainly drifting gratings with different orientations, in order to gain information about the selectivity of the excitatory and inhibitory signals.
Our results reveal a striking dissociation between the spatial and functional organization of excitation and inhibition. Calcium signals, reflecting excitatory activity, were sparse and selective, consistent with tuned responses to specific visual features. In contrast, chloride signals, reflecting inhibitory input, were broadly distributed and comparatively homogeneous across the neuronal population. Furthermore, calcium and chloride signals exhibited distinct temporal dynamics. Calcium transients were characterized by fast kinetics, reflecting rapid and transient excitatory activity. In contrast, chloride responses were slower and more sustained, with reduced amplitude and prolonged decay kinetics.
Together, these findings provide direct experimental evidence that excitation and inhibition are organized according to distinct population dynamics, with inhibition acting as a diffuse and coordinating signal and excitation encoding specific sensory information. This work highlights the importance of simultaneously measuring both components to understand cortical computation and demonstrates the power of chloride imaging as a tool to investigate inhibitory processes in vivo.