Seasonal affective disorder, or SAD, strikes millions of people each winter, bringing on feelings of sadness during colder months. Now, researchers believe they have uncovered the biochemical changes that bring on the common psychological condition.

University of Copenhagen investigators discovered that people who suffer from SAD process the neurotransmitter serotonin differently from the general population.

Seasonal affective disorder strikes in autumn, as daylight hours begin to shorten. In far-northern latitudes, the condition strikes up to one in six people.

Researchers studied 11 people experiencing seasonal affective disorder, using positron emission tomography (PET scans), along with 23 healthy subjects, serving as a control. Analysis showed those who experienced symptoms of the condition produced a greater amount serotonin transporter protein (SERT) in the winter than control subjects. This chemical suppresses serotonin, which can lead to feelings of sadness.

Serotonin transporter protein levels in SAD sufferers were an average of five percent higher in winter than in summer. Those concentrations remained largely unchanged year-round in subjects who did not experience symptoms.

"We believe that we have found the dial the brain turns when it has to adjust serotonin to the changing seasons... the higher the SERT activity the lower the activity of serotonin. Sunlight keeps this setting naturally low, but when the nights grow longer during the autumn, the SERT levels increase, resulting in diminishing active serotonin levels," Brenda McMahon of the University of Copenhagen said.

Some drugs called Serotonin Reuptake Inhibitors (SSRI's), developed to treat depression, work by regulating the flow of serotonin, retaining the neurotransmitter where it is most effective. One of the best-known SSRI drugs is fluoxetine, commonly marketed under the brand name Prozac. The drug was first developed in 1974, and was approved by the Food and Drug Administration in 1987.

Seasonal affective disorder is experienced by 9.7 percent of New Hampshire residents, and just 1.4 percent of those in Florida. The condition can bring on feelings of lethargy which drive some people toward excessive sleep. Some people may also overeat, especially carbohydrates, that can lead to weight gain, which can compound feelings of sadness. When sunlight returns in the spring, symptoms usually clear up. Psychiatrists do not classify SAD as a distinct disorder, but a form of major depressive episode.

Light therapy is often used to treat those experiencing sadness from the condition. In extreme cases, some patients are prescribed melatonin, a hormone, to balance brain chemistry.

Serotonin and its role on seasonal affective disorder was delivered at the 27th ECNP Congress, held in Berlin, Germany. That conference, sponsored by the European College of Neuropsychopharmacology, attracts thousands of mental health professionals each year.

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