Which neurotransmitter functions as both a neurotransmitter and a hormone and is essential for triggering fight or flight responses?

Epinephrine is the correct answer because it serves a dual role in the body, functioning both as a neurotransmitter in the nervous system and as a hormone in the endocrine system. It is produced by the adrenal glands and released into the bloodstream during stressful situations, triggering the physiological changes associated with the "fight or flight" response. This response includes increased heart rate, dilated airways, and enhanced blood flow to muscles, preparing the body for immediate action.

In addition to its hormonal role, epinephrine acts as a neurotransmitter in the brain, facilitating communication between nerve cells and playing a part in mood regulation and alertness. This unique capacity to operate as both a neurotransmitter and a hormone is what makes it critical in stress responses.

Other neurotransmitters listed, while significant in their functions—such as norepinephrine, acetylcholine, and GABA—do not exhibit the same dual role as epinephrine.