Prefrontal cortical regulation of brainwide circuit dynamics and reward-related behavior (2016)

Science 1 January 2016: Vol. 351 no. 6268

DOI: 10.1126/science.aac9698

Abstract

Motivation for reward drives adaptive behaviors, whereas impairment of reward perception and experience (anhedonia) can contribute to psychiatric diseases, including depression and schizophrenia. We sought to test the hypothesis that the medial prefrontal cortex (mPFC) controls interactions among specific subcortical regions that govern hedonic responses. By using optogenetic functional magnetic resonance imaging to locally manipulate but globally visualize neural activity in rats, we found that dopamine neuron stimulation drives striatal activity, whereas locally increased mPFC excitability reduces this striatal response and inhibits the behavioral drive for dopaminergic stimulation. This chronic mPFC overactivity also stably suppresses natural reward-motivated behaviors and induces specific new brainwide functional interactions, which predict the degree of anhedonia in individuals. These findings describe a mechanism by which mPFC modulates expression of reward-seeking behavior, by regulating the dynamical interactions between specific distant subcortical regions.

A way to modulate reward-seeking

Which brain regions are causally involved in reward-related behavior? Ferenczi et al. combined focal, cell type-specific, optogenetic manipulations with brain imaging, behavioral testing, and in vivo electrophysiology (see the Perspective by Robbins). Stimulation of midbrain dopamine neurons increased activity in a brain region called the striatum and was correlated with reward-seeking across individual animals. However, elevated excitability of an area called the medial prefrontal cortex reduced both striatal responses to the stimulation of dopamine neurons and the behavioral drive to seek the stimulation of dopamine neurons. Finally, modulating the excitability of medial prefrontal cortex pyramidal neurons drove changes in neural circuit synchrony, as well as corresponding anhedonic behavior. These observations resemble imaging and clinical phenotypes observed in human depression, addiction, and schizophrenia.

INTRODUCTION

The drive to seek and experience reward is conserved across species and, in mammals, involves interactions between subcortical dopaminergic systems and limbic structures such as the striatum. Impairment of this process, observed across a number of psychiatric conditions, is the clinical symptom of anhedonia (loss of enjoyment). The neural mechanisms underlying anhedonia are unknown but could result from abnormal interactions between cortical and subcortical reward circuits. We sought to test the hypothesis that elevated medial prefrontal cortex (mPFC) excitability (a clinical feature associated with anhedonia) exerts suppressive control over the interactions between two distant subcortical regions: the dopaminergic midbrain and the striatum.

RATIONALE

Clinical imaging studies have detected elevated activity in the mPFC in human patients with depression, and treatment is associated with normalization of this overactivity and improvement of anhedonic symptoms. Additionally, human studies have identified areas of the brain that respond to reward anticipation and experience, and this response can be suppressed in psychiatric disease. However, the source of this reward signal and the mechanisms underlying its modulation have not been causally demonstrated. We have integrated a diverse set of chronic and acute optogenetic tools with functional magnetic resonance imaging (fMRI) to provide a bridge between the causal, cellular specificity of rodent optogenetics and the brainwide observations that characterize human neuroimaging, with the goal of locally manipulating and globally visualizing neural activity to understand the regulation of reward-seeking behavior.

RESULTS

We demonstrate that stimulation of midbrain dopamine neurons drives both striatal fMRI blood oxygen level–dependent (BOLD) activity and reward-seeking behavior, and we show that these are correlated across individuals. We additionally find that silencing of dopamine neurons suppresses activity in the striatum, as well as in other brain regions (such as the hypothalamus), and drives avoidance behavior. Having established this bidirectional control of reward-seeking behavior, we then tested for perturbation of this circuitry via elevation of mPFC excitability. We observed suppression of striatal responses to dopamine, as well as the behavioral drive to seek out dopamine neuron stimulation and other natural rewarding stimuli. Finally, we demonstrate that stably elevated mPFC excitability synchronizes corticolimbic BOLD and electrophysiological activity, which in turn can predict anhedonic behavior in individual animals.

CONCLUSION

Our findings from experiments involving local cell-specific control, simultaneously with global unbiased observation of neural activity, reveal that the mPFC exerts top-down control over midbrain dopaminergic interactions with the striatum and that, when elevated, activity in the mPFC can suppress natural reward-related behavior. Furthermore, we observe that cortical-subcortical neural dynamics work in concert to regulate reward processing. All of these findings have implications for our understanding of natural reward-related physiology and behavior, as well as the pathogenesis of anhedonia.

Optogenetic fMRI was used to locally manipulate and globally visualize brainwide neural activity related to reward. Habituated rats were scanned in the awake state (top photographs). We establish that striatal BOLD activity is increased by optogenetic stimulation of dopamine neurons and decreased by optogenetic neural silencing. We demonstrate that focally elevated mPFC excitability suppresses reward-seeking behavior by exerting top-down control over striatal dopamine-induced activity and drives synchrony between specific corticolimbic circuits.