Despite the fact that layer 2/3 pyramidal neurons have been considered to be a relatively homogeneous cell population, it is well established that in vivo firing rates among these cells can vary more than ten-fold. Similarly, it has long been noted that a subset of neurons in the neocortex exhibit expression of the activity-dependent gene c-fos even under basal conditions. Here we show that a history of immediate-early gene expression is an indicator of elevated spontaneous firing activity in a subpopulation of layer 2/3 pyramidal neurons. Whether elevated activity is a cause or an effect of activated IEG expression,

these data indicate that some neurons disproportionately contribute to the propagation of neocortical activity. It has recently been proposed Temsirolimus research buy that a single extra

spike within a neocortical neuron might be capable of driving dozens of spikes in its synaptically connected partners ( London et al., 2010), and indeed, under some conditions stimulation of a single neurons can alter global network activity ( Li et al., 2009) and perception ( Houweling and Brecht, 2008). Based upon this finding, we propose that fosGFP+ neurons may Selleckchem PLX-4720 drive or propagate network activity and information transfer across brain areas. Is sensory input required to drive fosGFP expression and indirectly, the increased spontaneous firing activity of these cells? Our preliminary analysis

suggests this is not the case. Bilateral removal of all large facial vibrissae for 24 hr did not eliminate or even noticeably reduce fosGFP expression in layer 2/3. In addition, paired-cell recordings showed that fosGFP+ cells maintained elevated firing activity in sensory-deprived tissue (data not shown). Although it is possible that whisker removal is not sufficient to get rid of all afferent activity, these data suggest that sensory input is not required for fosGFP expression or elevated spontaneous firing. The question of whether these cell assemblies are generated from internal neocortical dynamics or are constructed by information from the periphery (Kenet et al., 2003, MacLean et al., 2005 and Golshani TCL et al., 2009) is of great interest. It has been suggested that the population of neurons exhibiting both high and low levels of activity are unstable and drift over timescales ranging from seconds to minutes (Ikegaya et al., 2004, Kerr et al., 2007 and Mokeichev et al., 2007). Our data indicate that the firing output of a cell is much more conserved than previously estimated. Due to the time course of fosGFP expression, which requires at least 2–3 hr to become fluorescently visible, we conclude that a subpopulation of neurons exhibits specific network connectivity, driving elevated activity that can be maintained for at least 4–7 hr.