Review Question - QID 109309

A 27-year-old woman presents to her primary care physician’s office complaining of trouble sleeping. She reports that for the past 10 months she has experienced difficulty falling asleep due constant worrying. The content of her worry include items such as whether or not her child will feel lonely in day care and the health of her parents. These worrying episodes typically begin toward the end of the day and last for several hours. She states that she has trouble concentrating at work as well and describes her heart as "racing" during these episodes. She denies any alcohol or illicit drug use. Which of the following neurotransmitters is most likely decreased in this patient?