A growing body of work indicates this may be, at least in part, a mechanism of action of the selective serotonin reuptake inhibitors (SSRIs) such as Prozac and Paxil.

Earlier work indicated that SSRIs stimulate growth of new brain cells, but researchers were unsure if this was simply a side effect of the drugs, rather than the mechanism by which the drugs treat depression.

Current theory holds that these medications block the reuptake of the neurotransmitter serotonin at nerve junctions, thereby increasing the store of serotonin available to affect and, ideally, improve mood.

René Hen and colleagues at Columbia University irradiated the hippocampi of mice to prevent new neuron growth.

When the mice were then given antidepressants, they no longer responded to the drugs by generating new neurons as did control mice.

The scientists concluded that the behavioral effects of SSRIs may be mediated by neuronal growth.

Adding weight to their observations and theory is the fact that in humans, antidepressants often take a month or more to work.

The laboratory results suggest that neurons need time to develop and grow in response to these drugs, if indeed this is a mechanism of action.

It also implies that no drug that works by this pathway will offer a quicker response than that produced by the current generation of medications.

This is terribly important, as many patients are profoundly depressed, often suicidal, when begun on these drugs, and a much more rapid effect would be enormously beneficial in preventing many tragic outcomes.