Several neurotransmitters (chemical messengers) in distinct areas of the brain have been demonstrated to play a role in the neurobiology of fear and anxiety [14]. Long-term dysregulation of these substances have effects on cortical and subcortical areas, and this appears to contribute to the development of anxiety disorders [14]. Research in animal physiology and human pharmacological studies have indicated that the familiar neurotransmitters ?-aminobutyric acid (GABA), norepinephrine, and serotonin are involved in anxiety disorders [15]. Other neurotransmitters and neuropeptides that interact and modulate fear and anxiety include CRF, neuropeptide Y, galanin, substance P, a variety of opioids, dopamine, glutamate, amino acid transmitters, and adrenal steroids such as cortisol [14–16]. Cholecystokinin (CCK), a neuropeptide found in the gastrointestinal (GI) tract and the brain, is the only circulating endogenous peptide that is known to be anxiogenic in humans [14, 15].

These neurochemicals, which work in various systems, play important adaptive roles in responding to stress, which include affecting energy stores, attention, vigilance, memory, planning, and cardiovascular function [14, 16]. Chronic activation, however, can be problematic. Clinically, several classes of medications are available that affect many of the neurotransmitters and are helpful in managing the primary anxiety disorders. These include the benzodiazepines, serotonin-1A agonists, and antidepressants affecting serotonin and norepinephrine [4].