Little is known about what causes people to exhibit improper fear behaviors in clinical disorders such as posttraumatic stress disorder (PTSD) or generalized anxiety disorder (GAD). The neural circuits involved in motivation, emotion, and attention all contribute to the expression of fear and defensive responding; dysfunction in any of these circuits may result in the inappropriate fear behaviors seen in PTSD and GAD. By examining specific brain areas implicated in fear and fear expression, we may gain a clearer understanding of the neural circuitry underlying anxiety-related behaviors in both normal and pathological instances. My work under the EAPSI fellowship was intended to incite an international collaboration between my lab at UCLA and my Korean lab at Ajou University. My project objective was to challenge the standard fear circuitry model; it is assumed that the amygdala (AMG) assigns aversive value (i.e., fear) to stimuli predicting danger. Neural projections from AMG to periaqueductal gray (PAG) are thought to convert fear into appropriate behavioral actions (i.e., fear expression). Recently it has been suggested that medial prefrontal cortex (mPFC) may play an important role in fear circuitry; clinical studies have shown that abnormal activity in mPFC is associated with the prevalence of some anxiety disorders. Recent animal studies have suggested that mPFC acts as the main inhibitory signal controlling fear "extinction". Based on evidence from my own lab, however, I proposed an alternative: mPFC works as a short term "working-memory" for fear. This novel fear model asserts that activity in mPFC may directly modulate activity in regions of dorsal and ventral PAG, causing a variety of defensive behaviors that are specific to the current level of threat (in this project, rats freeze during low threat but flee in response to more immediate threat). To test the proposed fear circuitry model, bilateral recording electrodes were implanted into brain areas of the rat that are believed to mediate fear (i.e., mPFC and PAG). While recording activity of single cells in each of these areas during a fear conditioning task, the AMG, which assigns motivational value to stimuli (and therefore is necessary for fear processing) was pharmacologically inactivated. Changes in neural activity in mPFC and PAG as a result of AMG inactivation were recorded. Data collected thus far support this novel fear model. In accordance with past studies, neuronal activity in dorsal and ventral PAG correlated with flight and freezing behavior, respectively. In support of my proposed hypothesis, neurons recorded in mPFC changed their activity as a function of PAG activation and therefore, defensive behavior. As predicted, mPFC activity increased from baseline firing during freezing expression (correlated with ventral PAG activation). And, mPFC activity increased at an even higher rate during flight behavior (correlated with an increase in dorsal PAG activity). Importantly, pharmacological inactivation of the AMG (which impairs fear processing) not only attenuated fear-evoked freezing and flight behaviors, but it also eliminated the increased activity seen in mPFC and PAG during fear behavior expression, suggesting that AMG-mediated fear behavior may be modulated by both mPFC and PAG. While data collected support the model I proposed, it will be necessary to record neurons from one or two more animals before a manuscript can be written and submitted for publication. Intellectual Merit. These findings provide insight into the neural circuitry regulating both normal and pathological fear states. Gaining an understanding about the contribution of brain areas not normally implicated in the "standard" fear model may change the way in which fear circuitry is currently conceptualized. These findings may lead to the revision of a widely accepted, yet possibly incorrect model system, which may eventually enable the development of new treatment approaches for anxiety disorders. Broader Impacts. It is possible that results from this project may bring new insight to the ways anxiety disorders are treated clinically. This project promoted collaboration between two diverse labs within the behavioral neuroscience community. Participating in this project provided an opportunity for me to help broaden the representation and participation of women in science. I am very grateful for the advances women have made insofar as striving to become represented equally in the workplace, and I hope my actions will help foster future participation of women in science. By helping to identify the specific neural correlates of fear, we may begin to better understand the circuitry mediating fear-related psychopathologies. Drug therapy to treat the symptoms of anxiety-related disorders is common in clinical practice, but not until the exact mechanisms underlying fear (and inappropriate fear) are identified can we properly treat the disorders themselves. These findings may help re-define the current model of fear circuitry, which would lay vital groundwork for understanding clinical disorders in the context of biological mechanisms.
