Functional Imaging of Ventral Tegmental Area Dopamine Neurons during Complex Reward-Learning Behavior

Abstract

The dopamine system, located primarily in the ventral tegmental area (VTA) and the substantia nigra, plays an important role in the reward-learning of both humans and mice. These dopamine neurons do not fire when a reward is received, but rather when there is a prediction error between what is expected and what occurs. Our understanding of dopamine neurons’ role in reward-learning and working memory has been established through short single cell recordings and pharmacological studies, but never through functional imaging of the relevant brain areas. This is a result of the depth of the ventral tegmentum and the inadequacies of the technology until now. This study carries out high-resolution functional imaging of ventral tegmentum dopamine neurons by inserting a cre-dependent GCaMP virus into the VTA of DAT-cre mice and then implanting a Gradient Index lens into the VTA to allow for two-photon microscopy. The VTA is imaged while completing a complex reward learning task in a virtual reality environment that simulates running down a T-stem maze. The raw data is then motion corrected using a unique algorithm that corrects for both fast jitter and slow drift movements, which allows single neurons to be recorded for extended periods of time. This functional imaging directly demonstrates that dopamine neurons follow the basic features of the traditional dopamine response, as the response shifts from reward to cue over the course of training and also diminishes as the reward becomes more expected. Another interesting finding is that single neurons are selective for specific visual cues during the task, but the selectivity of the neuron is not constant and can shift over time. These findings demonstrate the value of functional neuronal imaging in deep brain areas during complex behavior.