Intrinsic excitability mechanisms of neuronal ensemble formation

Neuronal ensembles are coactive groups of cortical neurons, found in both spontaneous and evoked activity, which can mediate perception and behavior (Cossart et al., 2003; Buzsaki, 2010; Carrillo-Reid et al., 2019; Marshel et al., 2019). To understand the mechanism that lead to the formation of neuronal ensembles, we generated optogenetically artificial photo-ensembles in layer 2/3 pyramidal neurons in brain slices of mouse visual cortex from both sexes, replicating an optogenetic protocol to generate ensembles in vivo by simultaneous coactivation of neurons (Carrillo-Reid et al. 2016). Using whole-cell voltage-clamp recordings from individual neurons and connected pairs, we find that synaptic properties of photostimulated were surprisingly unaffected, without any signs of Hebbian plasticity. However, extracellular recordings revealed that photostimulation induced strong increases in spontaneous action potential activity. Using perforated patch clamp recordings, we find increases in neuronal excitability, accompanied by increases in membrane resistance and a reduction in spike threshold. We conclude that the formation of neuronal ensemble by photostimulation is mediated by cell-intrinsic changes in excitability, rather than by Hebbian synaptic plasticity or changes in local synaptic connectivity. We propose an "iceberg" model, by which increased neuronal excitability makes subthreshold connections become suprathreshold, increasing the functional effect of already existing synapses and generating a new neuronal ensemble.