Enhanced Aversive Signals During Classical Conditioning in Dopamine Axons in Medial Prefrontal Cortex

Abstract

Midbrain dopamine neurons impact neural processing in the prefrontal cortex (PFC) through mesocortical projections. However, the signals conveyed by dopamine projections to the PFC remain unclear, particularly at the single-axon level. Here, we investigated dopaminergic axonal activity in the medial PFC (mPFC) during reward and aversive processing. By optimizing microprism-mediated two-photon calcium imaging of dopamine axon terminals, we found diverse responses in dopamine axons, with some preferring reward and others preferring aversive stimuli, with a strong bias for the latter at the population level. Long-term longitudinal imaging revealed that the preference was maintained in reward- and aversive-preferring axons throughout classical conditioning in which rewarding and aversive stimuli were paired with preceding auditory cues. However, as mice learned to discriminate reward or aversive cues, a cue activity preference gradually developed only in aversive-preferring axons, becoming more selective for aversive processing. We inferred the trial-by-trial cue discrimination based on machine learning using anticipatory licking or facial expressions, and found that successful discrimination was accompanied by sharper selectivity for the aversive cue in aversive-preferring axons. Our findings implicate mesocortical dopamine axon activity in the encoding of aversive processing that is modulated by both classical conditioning across days and trial-by-trial discrimination within a day. Two-photon calcium imaging revealed that many mesocortical dopamine axons show enhanced selectivity for aversive cue processing during classical conditioning.