What if everything we thought we knew about anxiety disorders is wrong? A recent study of serotonin, the chemical responsible for your mood, suggests that the group of conditions currently identified as anxiety disorders may need to be recategorized.

The identification and grouping of mental disorders has come a long way in a relatively short period. In terms of classifying disorders, the last hundred years have been the most important time in history, and the time during which we’ve advanced the most in the organization and treatment of mental disorders. Nevertheless, like any field of science, there are still improvements to be made when it comes to naming and differentiating between disorders.

What Are Anxiety Disorders?

Anxiety disorders are a grouping of several mental conditions whose symptoms cluster around “excessive, irrational fear, and dread,” according to the National Institute of Mental Health. Anxiety disorders are the most common kind of emotional disorders, affecting about 25 million Americans. The grouping generally includes panic disorders, obsessive compulsive disorder (OCD), phobias (including social phobia), and generalized anxiety disorder (GAD). Besides the aforementioned irrational and excessive fear, other symptoms commonly include intrusive memories, obsessive thoughts, and physical symptoms such as nausea and “butterflies.”

The conditions are all grouped under the “anxiety disorder” umbrella, despite having some serious differences in types of symptoms, duration of symptoms, and triggers of symptoms.

How Does Serotonin Fit In?

Serotonin is a chemical found in the body that acts as a neurotransmitter — it carries a signal from one neuron to another. It is derived from tryptophan, a necessary amino acid in our diet, and is found in various parts of the body, including the gastrointestinal tract, blood platelets, and the central nervous system. Contrary to popular belief, the majority of our body’s serotonin is actually located in the gastrointestinal tract (about 80 to 90 percent of the body’s supply), rather than the brain.

In discussing serotonin’s relationship to anxiety and fear disorders, the serotonin in the brain will be the focus. Serotonin is believed to have many effects on our psychological state. It has a very wide distribution in the brain, affecting many different areas that correlate to many different parts of our functioning. Being so ubiquitous, serotonin is believed to have an influence on aspects of our brain functioning that include mood, social behavior, appetite, sexual desire and functioning, sleep, and memory.

Some researchers believe there’s a link between serotonin’s function in the brain and the development of mental disorders. Possible contributors to mental disorder development include the brain producing lower-than-normal levels of serotonin, a lack of receptor sites able to receive the serotonin that has been produced, a problem in serotonin transport, or a lack of tryptophan, the chemical from which serotonin is derived.

Anxiety disorders in particular have been strongly associated with low levels of serotonin. However, it is still difficult to say for certain that inadequate serotonin levels cause anxiety. It could very well be the other way around — anxiety disorders causing lower levels of serotonin. Another factor muddling conclusions is that it is currently impossible to measure the serotonin levels in the brain. We can measure the level of the neurotransmitter in our blood, but researchers aren’t sure if blood levels of serotonin reflect levels in the brain.

Despite these uncertainties, selective serotonin reuptake inhibitors (SSRIs) are commonly prescribed to patients with anxiety disorders and other mental health conditions. Including popular brands like Prozac and Zoloft, SSRIs work by stopping nerves from reabsorbing serotonin, thereby making the chemical more available to boost mood.

So What’s The New Information, And Why Is It Important?

According to a team of international researchers, the functioning of the serotonergic system is not homogenous in the group of disorders called anxiety disorders. Published in the Journal of Psychopharmacology, the research analyzed six studies that evaluated the effect of reducing tryptophan (the precursor to serotonin) in patients that had received treatment for an anxiety disorder. This paradigm often results in a severe reduction of serotonin levels in the body, allowing researchers to make observations about the importance of the neurotransmitter in disorders and psychological functioning.

The researchers predicted that reductions in serotonin levels would cause a worsening of symptoms in those patients with a disorder more related to fear (a phobia, for example), but not in those with a disorder more related to anxiety (general anxiety disorder), even though they’re all currently grouped together and treated with SSRIs. Their reanalysis of studies directly confirmed this theory.

"The idea of responses to threatening stimuli causing feelings and emotions related to fear and anxiety, as well as a myriad of subgroups within these responses, is not new,” said Felipe Corchs, of the Institute and Department of Psychiatry, School of Medicine, University of São Paulo in a press release. “However, our study gives an important step toward sub clustering of disorders once it is based on one of the most important neurotransmitters involved in these reactions and in the fact that it was tested in actual psychiatric patients.”

The study authors say the distinction is important not only to ongoing efforts to improve the categorization of mental disorders, but also to the development of new treatments.

“Further studies need to be developed with specific methodology to identify the exact clinical characteristics of patients who have these two pharmacological profiles independently of the current diagnostic criteria of the DSM and ICD,” Corchs said.” This may contribute to future etiology-based diagnostic criteria.”

Source: Cochs F, Nutt D, Hince D, Davies S, Bernik M, Hood S. Evidence For Serotonin Function As A Neurochemical Difference Between Fear And Anxiety Disorders In Humans? Journal of Pharmacology. 2015.