Regulation of synaptic connections by intrinsic electrical activity.    Newly-generated neurons (green) in adult mice are rendered hyperexcitable by delivering into them Nachbac, a voltage-gated sodium channel, via recombinant retroviruses. (Bottom) Genetically-enhanced excitability increases the number of inhibitory synapses (arrows) on the genetically modified neurons (green). By genetically controlling the electrical properties of neurons we investigate how neuronal activity regulates the integration of cells into brain circuits, and the connections between neurons.