SYNERGY AND ANTAGONISM OF THE CHOLINERGIC AND DOPAMINERGIC SYSTEMS DURING ASSOCIATIVE LEARNING

Neuromodulatory systems play a critical role in learning by reinforcement. However, whether and how they cooperate or compete to jointly control associative learning functions remained elusive. Here, we show that dopamine and acetylcholine act both synergistically and antagonistically to signal prediction errors, central for reinforcement learning models. We recorded cholinergic and dopaminergic spiking activity or the release of acetylcholine and dopamine neurotransmitters concurrently while mice (n = 38) performed an operant auditory discrimination task. Trial-averaged electrophysiologically recorded single neuron activity and fiber photometry recordings of transmitter release revealed robust parallel representations of reward prediction error (RPE) after rewarding and reward-predicting stimuli, suggesting synergistic effects. However, aversive outcomes elicited opposite responses in cholinergic and half of the dopaminergic neurons, and trial-by-trial negative correlations of punishment responses of those neuron types suggested an antagonistic relationship. Potentially underlying this, channelrhodopsin-assisted circuit mapping revealed a disynaptic inhibitory pathway from cholinergic neurons to a population of midbrain dopaminergic cells via excitation of midbrain-projecting basal forebrain inhibitory neurons. Finally, chemogenetic inhibition of cholinergic neurons disrupted dopaminergic RPE representations and learning behavior, while it reduced punishment-induced suppression of dopamine release, demonstrating the cell-type- and temporally specific dual presence of synergistic and antagonistic neuromodulator effects. These results reveal tight and specific coordination between two major neuromodulatory systems, the activities and functions of which have hitherto largely been interpreted independently.