A spatiotemporal orchestration of balanced cholinergic effects regulates cortical activation

During arousal, attention, and memory formation, functionally relevant areas of the neocortex enter an “activated” state marked by desynchronized activity. Previous pharmacological experiments have shown that acetylcholine plays a fundamental role in cortical activation via muscarinic receptors. However, muscarinic receptors have diverse and sometimes contradictory effects in different cell types and cortical layers, and as a result the mechanism(s) of this activating influence remain unclear. Computational models have predicted that increasing excitatory neuron excitability (an M1-type muscarinic receptor-mediated effect) drives activation, while synaptic adaption (an M2-type mediated effect) sustains the activated state, but this is yet to be tested in an experimental preparation. To investigate how these diverse cellular actions mediate the effects on cortical activation, we generated cell-type- and receptor-type-specific conditional knockout mice for M1- and M2-type muscarinic receptors and used high-density silicon probes to record all layers of the somatosensory cortex while monitoring whisker movement and pupil size. We observed that cortical activation can vary in both the degree of activation, and its spatiotemporal spread through the cortical column. While large pupil dilations were coupled to whisking, acetylcholine release, and column-wide cortical activation, small dilations were associated only with localized activation in the infragranular layers. Remarkably, we found that ablating M2-type signaling on excitatory synapses facilitated the propagation of the activated state throughout the cortical column, instead of the activated state collapse predicted in silico. Ablating M1-type signaling on excitatory or dendritic-targeting inhibitory neurons attenuated or enhanced the magnitude of activation, respectively. These opposing effects of M1-type signaling shifted the baseline cortical state from an infragranular delta rhythm (which suppressed activation) to a supragranular one (which facilitated activation), suggesting that acetylcholine plays a push-pull, balancing role on cortical activation. In summary, we reveal an orchestration of cholinergic-mediated processes that promote and control the spatiotemporal dynamics of cortical activation.