Characterization of glutamatergic VTA neural population responses to aversive and rewarding conditioning in freely-moving mice.

The Ventral Tegmental Area (VTA) is a midbrain structure known to integrate aversive and rewarding stimuli, but little is known about the role of VTA glutamatergic (VGluT2) neurons in these functions. Direct activation of VGluT2 soma evokes rewarding behaviors, while activation of their downstream projections evokes aversive behaviors. To facilitate our understanding of these conflicting properties, we recorded calcium signals from VTA neurons using fiber photometry in VGluT2-cre mice to investigate how this population was recruited by aversive and rewarding stimulation, both during unconditioned and conditioned protocols. Our results revealed that, as a population, VTA neurons responded similarly to unconditioned-aversive and unconditioned-rewarding stimulation. During aversive and rewarding conditioning, the CS-evoked responses gradually increased across trials whilst the US-evoked response remained stable. Retrieval 24 h after conditioning, during which mice received only CS presentation, resulted in VTA neurons strongly responding to CS presentation and to the expected-US but only for aversive conditioning. To help understand these differences based on VTA neuronal networks, the inputs and outputs of VTA neurons were investigated using Cholera Toxin B (CTB) and rabies virus. Based on our results, we propose that the divergent VTA neuronal responses to aversion and reward conditioning may be partly due to the existence of VTA subpopulations that are characterized by their connectivity.