Calcium is a universal messenger regulating essential cellular signaling events in many tissues and organisms. In neurons, action potentials trigger large and rapid changes in cytoplasmic-free calcium concentration. Genetically encoded calcium indicators (GECIs) have been iteratively improved and are becoming useful for quantitative imaging of activity in defined neuronal populations in vivo. Among them, GCaMP6m has been recently introduced as an ultrasensitive calcium sensor for the experiments in vivo. Whether GECIs can be used in developmental studies to characterize early activity patterns in the immature cerebral cortex remains unknown, however. We used in vivo two-photon calcium imaging with GCaMP6m calcium indicator to characterize its developmental expression following AAV mediated viral induction of GCaMP6m expression in newborn mice (at postnatal days P0-1). Three days after virus injection, the vast majority of neurons showed fluorescent labeling in the neuronal cytoplasm, dendrites and axons. We found that the GCaMP6m calcium sensor is expressed in the cerebral cortex within 3-5 days following transfection at levels that enable monitoring spontaneous and sensory-evoked calcium transients in the intact mice barrel cortex in vivo. Thus, following GCaMP6m transfection at birth, it is possible to monitor neuronal populations in the barrel field of somatosensory cortex in P5-7 mice in vivo.
The study was supported from FP7-ERC “GABA Networks” grant (# 242842) and Russian Federation grant № 11.G34.31.0075.