A collaborative study involving scientists from Georgetown University Medical Center and Robarts Research Institute in Canada has found that commonly prescribed SSRI drugs (selective serotonin reuptake inhibitors) may not only manipulate the neurotransmitter serotonin in the brain, but may also affect the human immune system in still undetermined ways.
The researchers are the first to find that serotonin is passed between key cells in the immune system, and that the chemical is specifically used to activate an immune response.
Presently unknown, however, is if SSRIs like Prozac, Zoloft, Paxil and others have a beneficial or a damaging effect on the human immune system. According to Gerard Ahern, Ph.D., assistant professor of Pharmacology at Georgetown and lead researcher on the study: "The wider health implication is that commonly used SSRI antidepressants, which target the uptake of serotonin into neurons, may also impact the uptake in immune cells."
Since “we just don’t know how these drugs might affect immunity we really need to clarify the normal role of serotonin in immune cell functioning," Ahern said. Thus, while the drugs may help to restore a healthy immune function in depressed patients who are also prone to infections, it is equally possible that SSRIs might bolster immunity to the point that they actually trigger autoimmune disease.
The findings of the study were published in the journal Blood last October and then highlighted in the December issue of Nature Reviews Immunology. Currently, the researchers are engaged in developing an animal model that may help clarify the exact nature of the interaction.
Serotonin transmission between neurons in the brain is associated with feelings of pleasure, mood, and appetite. SSRIs keep serotonin active within the synaptic spaces between neurons, enhancing the chemical’s positive effects.
In the immune system, rather than using chemical messengers to communicate between nerve cells, communication is thought to be through the physical contact of one type of immune cell touching another, thereby setting off a response.
According to Ahern: "In addition to the physical contact, it surprised us to find that these immune cells also have machinery to take up serotonin and to secrete it in an excitatory manner. The point behind this transmission is not entirely clear, but it appears to be an additional way of stimulating a T cell response."
Thus, Ahern theorizes that SSRIs that block serotonin reuptake "likely change some of the parameters of T-cell activation, but we don’t know yet if it enhances or inhibits the total immune response. But it is something that should be explored because we really have no idea what SSRIs are doing to people’s immune systems."