Intrinsic excitability mechanisms of neuronal ensemble formation

Neuronal ensembles are coactive groups of cortical neurons, found in spontaneous and evoked activity, that can mediate perception and behavior. To understand the mechanisms that lead to the formation of ensembles, we co-activated optogenetically and electrically layer 2/3 pyramidal neurons in brain slices from mouse visual cortex, in animals from both sexes, replicating in vitro an optogenetic protocol to generate ensembles in vivo. Using whole-cell and perforated patch-clamp pair recordings we find that, after optogenetic or electrical stimulation, coactivated neurons increase their correlation in spontaneous activity, a hallmark of ensemble formation. Coactivated neurons showed small biphasic changes in presynaptic plasticity, with an initial depression followed by a potentiation after a recovery period. Unexpectedly, optogenetic and electrical stimulation-induced significant increases in frequency and amplitude of spontaneous EPSPs, even after single-cell stimulation. In addition, we observed strong and persistent increases in neuronal excitability after stimulation, with increases in membrane resistance and reduction in spike threshold. A pharmacological agent that blocks changes in membrane resistance can revert this effect. These significant increases in excitability may partly explain the observed biphasic synaptic plasticity. We propose that cell-intrinsic changes in excitability are involved in the formation of neuronal ensembles. We propose an "iceberg" model, by which increased neuronal excitability makes subthreshold connections suprathreshold, enhancing the effect of already existing synapses, and generating a new neuronal ensemble. Significance StatementWe investigated the synaptic and cellular mechanisms underlying the formation of neuronal ensembles, i.e., spontaneously coactive groups of neurons. Using in vitro electrophysiology and optogenetic in slices of mouse neocortex we replicated a protocol that generates ensembles in vivo. After optogenetic and electrical stimulation, we observed biphasic synaptic plasticity and, unexpectedly, major increases in excitability, input resistance, and reductions in firing threshold. The increased excitability can explain the observed synaptic plasticity. Our results reveal a major role for intrinsic excitability in the establishment of neuronal ensembles.