People suffering from depression do not have enough receptors to suitably respond to the brain's feel-good chemicals, according to a new study conducted by The University of Michigan Depression Centre. This is perhaps why some patients do not respond positively to treatment while some others do.
Researchers said that even among depressed people, the numbers of these receptors could vary greatly.
"There's a substantial amount of biological difference even among people who have major depression, which is just as important as the biological differences between people with depression and people without," said lead U-M researcher, Jon-Kar Zubieta, M.D., Ph.D.
"The more we can understand about these differences, the better we can address treatment to the individual and have the greatest effect on symptoms," he added.
Zubieta and his team conducted positron emission tomography (PET) scans on the brains of patients diagnosed with major depression but had not yet received treatment for it.
They were looking for the 5HT1 receptor, which allows brain cells to receive signals from serotonin, one of the chemicals that the brain produces to transmit mood-related signals.
The scans were then compared to the scans of non-depressed patients.
Researchers found that levels of 5HT1 receptors were considerably lower in the brains of depressed people than in non-depressed people.
In fact, the lower the 5HT1 levels, the more poorly the subjects scored on tests of their ability to engage in daily activities.
As well, the lower the 5HT1 levels, the less effective antidepressants were in relieving depression symptoms.
While scientists know that serotonin levels are linked to depression, not as much research has been done on 5HT1-receptor levels and how they might affect the disease.
That's why researchers chose to scan only people who had not yet received antidepressant medications, since some such medications might actually encourage the brain's cells to make more serotonin receptors - and masking the actual level of receptors that the person has naturally.
Zubieta said that this finding of individual variation might help explain why in current depression treatment, some patients find great relief from a medication that doesn't help other equally depressed patients.
In another segment of the study, both depressed and non-depressed subjects received PET scans that looked for mu-opioid receptors in their brains.
These receptors receive signals from chemicals known as endogenous opioids, also known as endorphins, which function as the brain's natural painkillers.
They are also known as 'feel-good' chemicals that help create a natural high.
As was the case with 5HT1 receptors, depressed study subjects had lower levels of mu-opioid receptors, and the fewer they had, the less likely they were to benefit from an antidepressant.
The findings were presented at the American Psychiatric Association's annual meeting in Washington, D.C.