Brain Circuits Mediating Opposing Effects on Emotion and Pain

The amygdala is important for processing emotion, including negative emotion such as anxiety and depression induced by chronic pain. Although remarkable progress has been achieved in recent years on amygdala regulation of both negative (fear) and positive (reward) behavioral responses, our current understanding is still limited regarding how the amygdala processes and integrates these negative and positive emotion responses within the amygdala circuits. In this study with optogenetic stimulation of specific brain circuits, we investigated how amygdala circuits regulate negative and positive emotion behaviors, using pain as an emotional assay in male rats. We report here that activation of the excitatory pathway from the parabrachial nucleus (PBN) that relays peripheral pain signals to the central nucleus of amygdala (CeA) is sufficient to cause behaviors of negative emotion including anxiety, depression, and aversion in normal rats. In strong contrast, activation of the excitatory pathway from basolateral amygdala (BLA) that conveys processed corticolimbic signals to CeA dramatically opposes these behaviors of negative emotion, reducing anxiety and depression, and induces behavior of reward. Surprisingly, activating the PBN-CeA pathway to simulate pain signals does not change pain sensitivity itself, but activating the BLA-CeA pathway inhibits basal and sensitized pain. These findings demonstrate that the pain signal conveyed through the PBN-CeA pathway is sufficient to drive negative emotion and that the corticolimbic signal via the BLA-CeA pathway counteracts the negative emotion, suggesting a top-down brain mechanism for cognitive control of negative emotion under stressful environmental conditions such as pain. It remains unclear how the amygdala circuits integrate both negative and positive emotional responses and the brain circuits that link peripheral pain to negative emotion are largely unknown. Using optogenetic stimulation, this study shows that the excitatory projection from the parabrachial nucleus to the central nucleus of amygdala (CeA) is sufficient to drive behaviors of negative emotion including anxiety, depression, and aversion in rats. Conversely, activation of the excitatory projection from basolateral amygdala to CeA counteracts each of these behaviors of negative emotion. Thus, this study identifies a brain pathway that mediates pain-driven negative emotion and a brain pathway that counteracts these emotion behaviors in a top-down mechanism for brain control of negative emotion.