Some people suffering from depression have fewer receptors for mood-boosting chemicals, a new study says, which could explain why some patients respond to treatment better than others.

Researchers at the University of Michigan Depression Center said that even among depressed people, the numbers of these receptors vary greatly.

Dr. Jon-Kar Zubieta and his team conducted positron emission tomography (PET) scans on the brains of patients diagnosed with major depression. 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 researchers only scanned patients who had been diagnosed but who had not yet received treatment. It is thought that anti-depressants may encourage the brain to create more 5HT1 receptors, which would mask how many these patients had naturally.

The scans were then compared to the scans of non-depressed patients.

The team 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.

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 this week at the American Psychiatric Association's annual meeting in Washington, D.C.