Title: Long-range circuits for learning of audiomotor tasks: investigating cholinergic control of sensorimotor acquisition 

During sensorimotor learning, animals link a sensory cue with actions that are separated in time using circuits distributed throughout the brain. Learning thus requires neural mechanisms that can operate across a wide spatiotemporal scale and promote learning-related plasticity. Neuromodulatory systems—with their broad projections and multiple timescales of activity—fulfill these criteria and could serve as a potent mechanism to link the different sensory and motor components. Yet, it remains unknown the extent to which this proposed model of plasticity occurs in real-time during behavioral learning. Acquisition of sensorimotor learning in a go/no-go task is much faster and more stereotyped than previously considered (Kuchibhotla et al., 2019). We trained mice to respond to one tone for a water reward (S+) and withhold from responding to another (S-). We interleaved reinforced trials with those where reinforcement was absent (“probe”). Early in learning, animals discriminated between S+ and S- in probe but not reinforced trials. This unmasked a rapid acquisition phase of learning followed by a slower phase for reinforcement, termed ‘expression’. What role does neuromodulation play in task acquisition? Here, we test the hypothesis that cholinergic neuromodulation provides a ‘teaching signal’ that drives primary auditory cortex (A1), and links stimuli with reinforcement. We exploit our behavioral approach and combine this with longitudinal two-photon calcium imaging of cholinergic activity in A1 during discrimination learning. We report both robust stimulus-evoked cholinergic activity to both S+ and S- and stable licking-related activity throughout learning at the level of the axon segment. While this activity mildly habituates in a passive control, in behaving animals the S+ and S- stimulus-evoked activity is enhanced (S+: duration, S-: amplitude and duration) during early learning. Additionally, we test the hypothesis that cholinergic neuromodulation impacts the rate of task acquisition. We bilaterally expressed ChR2 in cholinergic neurons within the basal forebrain of ChAT-cre mice and activated these neurons on both S+ and S- trials throughout learning. Test animals acquired the task faster than control groups as measured in probe trials. These results suggest that phasic bursts of acetylcholine, projecting widely to cortical regions, directly impact the rate of discrimination learning.