Monosynaptic ventral tegmental area glutamate projections to the locus coeruleus enhance aversive processing

Distinct excitatory synaptic inputs to the locus coeruleus (LC) modulate behavioral flexibility. Here we identify a novel monosynaptic glutamatergic input to the LC from the ventral tegmental area (VTA). We show robust VTA axonal projections provide direct glutamatergic transmission to LC. Despite weak synaptic summation, optogenetic activation of these axons enhances LC tonic firing and facilitates real-time and conditioned aversive behaviors. We hypothesized this projection may modulate synaptic integration with other excitatory inputs. We then used coincident VTA-LC photostimulation with local electrical stimulation and observed enhanced LC burst induction. To determine whether this integration also occurs in vivo, we took an analogous approach measuring reward-seeking behavior during unpredictable probabilistic punishment. Here, glutamatergic VTA-LC photostimulation during a concurrent noxious stimulus did not delay reward-seeking behavior, but increased probability of task failure. Together, we identified a novel VTA-LC glutamatergic projection that drives concurrent synaptic summation during salient stimuli to promote behavioral avoidance.