Neuronal networks in the spotlight

The brain’s astounding achievements regard­ing movement control and sensory process­ing are based on complex spatiotemporal ac­tivity patterns in the relevant neuronal net­works. Our understanding of neuronal net­work activity is, however, still poor, not least because of the experimental difficulties in di­rectly observing neural circuits at work in the living brain (in vivo). Over the last decade, new opportunities have emerged-especial­ly utilizing two-photon microscopy-to in­vestigate neuronal networks in action. Cen­tral to this progress was the development of fluorescent proteins that change their emis­sion depending on cell activity, enabling the visualization of dynamic activity patterns in local neuronal populations. Currently, genet­ically encoded calcium indicators, proteins that indicate neuronal activity based on ac­tion potential-evoked calcium influx, are be­ing increasingly used. Long-term expression of these indicators allows repeated moni­toring of the same neurons over weeks and months, such that the stability and plastici­ty of their functional properties can be char­acterized. Furthermore, permanent indicator expression facilitates the correlation of cel­lular activity patterns and behavior in awake animals. Using examples from recent studies of information processing in the mouse neo­cortex, we review in this article these fasci­nating new possibilities and discuss the great potential of the fluorescent proteins to eluci­date the mysteries of neural circuits.