Single‐Cell Calcium Imaging of RFP Labeled Interneurons in the Neocortex of Tbr1‐Deficient Neonatal mice

Cortical development involves initial structuring of network features genetically programmed signaling pathways. Later, ion channel activity helps refine neuronal connections. In this study we examine propagating waves of activity in a mouse genetically engineered to lack the transcription factor T‐box, brain 1 ( Tbr1 ), which is involved in the differentiation of cortical excitatory neurons and cortical patterning. Using calcium imaging, we find that wave activity appears at normal frequencies. Additionally we examine the distribution of cortical inhibitory neurons in Tbr1 deficient cortex using red‐fluorescent‐protein (RFP), and find that these neurons are displaced similarly to what is observed in reeler mice. Sensitivity of waves to blockade of GABA signaling is preserved in Tbr1 deficient mice. Finally, we show using dual green and red‐channel imaging of calcium indicator dye and RFP fluorescence, respectively, that inhibitory interneurons participate in spontaneous calcium signaling at normal frequencies in Tbr1 deficient mice. We identify two distinct types of spontaneous calcium signals: one that is consistent with wave activity, and another that is restricted to single cells and not correlated with neighboring cells, and sensitive to blockers of L‐type calcium channels. These results provide evidence that wave activity is a robust property of developing networks, and establish new methods of recording calcium signals from genetically labeled cell populations. We identify distinct calcium signals in immature interneurons that may serve separate purposes in development.