Highly active neurons emerging in vitro.

Abstract

Mean firing rates vary across neurons in a neuronal network. Whereas most neurons infrequently emit spikes, a small fraction of neurons exhibit extremely high frequencies of spikes; this fraction of neurons plays a pivotal role in information processing; however, little is known about how these outliers emerge and whether they are maintained over time. In primary cultures of mouse hippocampal neurons, we traced highly active neurons every 24 h for 7 weeks by optically observing the fluorescent protein dVenus; the expression of dVenus was controlled by the promoter of Arc, an immediate early gene that is induced by neuronal activity. Under default-mode conditions, 0.3-0.4% of neurons were spontaneously Arc-dVenus positive, exhibiting high firing rates. These neurons were spatially clustered, exhibited intermittently repeated dVenus expression, and often continued to express Arc-dVenus for approximately two weeks. Thus, highly active neurons constitute a few select functional subpopulations in the neuronal network.