Common antidepressants may affect immune system: Canada-U.S. study
LONDON, ON (Jan. 24, 2006) ? Drugs that treat depression by manipulating the neurotransmitter serotonin in the brain may also affect the users immune system, according to a joint Canada-U.S. study.
Thats because the investigators found, for the first time, that serotonin is passed between key cells in the immune system, and that the chemical is specifically used to activate an immune response. They do not know yet, however, whether SSRI (selective serotonin reuptake inhibitors) drugs including Prozac, Zoloft, Paxil and others could have either a beneficial or a damaging effect on human immunity.
The wider implication of these findings is that commonly used SSRI antidepressants, which target the uptake of serotonin into neurons, may also affect immune function, said immunologist Dr. Peta OConnell, a scientist at Robarts Research Institute in London and lead researcher on the study with pharmacologist Dr. Gerard Ahern of Georgetown University in Washington, D.C. The study is featured on the cover of the February edition of the journal Blood.
In the case of SSRIs, we just dont know how these drugs might affect immunity, so we really need to clarify the normal role of serotonin in immune cell functioning. There are reports in the medical literature indicating that dysregulated serotonin can affect immune function in both good and bad ways, said OConnell, a member of the BioTherapeutics Research Group at Robarts and also assistant professor in the Department of Microbiology and Immunology at The University of Western Ontario.
It brings to mind the adage a smile a day keeps the doctor away but these findings also raise questions about immune function for people who suffer chronic pain as serotonin is a key chemical signal in this process. In addition, it raises questions about immune function for those who use drugs of abuse such as methamphetamines, ecstasy and cocaine that are known to raise blood levels of serotonin.
The surprising finding that serotonin is rapidly passed between immune cells in a manner similar to its transmission between brain neurons was first published online in mid-October and in December the discovery was highlighted for the general scientific audience by the journal Nature Reviews Immunology.
In the brain, serotonin transmission between brain neurons is associated with feelings of pleasure, mood, appetite and pain, and the class of antidepressants known as SSRIs keeps serotonin active within the synaptic spaces between neurons, enhancing the chemicals positive effects.
Unlike in the brain, which uses chemical messengers to communicate between nerve cells, the immune system is believed to converse through physical contact ? one type of immune cell touches and/or sends chemicals to another, setting off a response. Specifically, antigen presenting cells display their antigens (bits of a foreign invader) to T-cells and the resulting physical coupling between the antigens and the T-cells will prompt the T-cells to divide and expand in number, triggering an immune response designed to destroy the invader. This process may take hours to days.
What this study found, however, is that dendritic cells ? the most powerful of the antigen-presenting cells and the ones that find microbes and foreign material entering the body and educate the immune system to fight them ? also use serotonin to quickly excite a T-cell response.
They discovered that these dendritic cells can rapidly secrete serotonin, which is then taken up by serotonin receptors on certain types of T-cells.
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, Ahern added. The point behind this transmission is not entirely clear, but it appears to be an additional way of stimulating a T-cell response.
Drugs that block serotonin reuptake likely change some of the parameters of T-cell activation, but we dont know yet if it enhances or inhibits the total immune response, Ahern said. But it is something that should be explored because we really have no idea what SSRIs are doing to peoples immune systems.
This study was funded by the U.S. National Institutes of Health, the Canada Foundation for Innovation, Ontario Innovation Trust and an Ontario Premiers Research Excellence Award.
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For more information, contact: Linda Quattrin, Robarts Research Institute, (519) 663-3021