If psychedelics heal, how do they do it?

I pass on from a recent issue of PNAS this informative "News Feature" article by Carolyn Beans (open source).  It discusses studies trying to determine the brain mechanisms by which therapeutically effective psychedelics such as MDMA (3,4-methylenedioxymethamphetamine, commonly known as ecstacy), psilocybin, and LSD have their effect.  They all act on serotonin receptors.