The general concern involving MDMA and brain damage is not whether serotonin levels are depleted permanently so much as whether the centers that produce the serotonin are damaged and cannot produce the serotonin anymore (or as much). Because your body SHOULD be able to create more serotonin otherwise.
Of course, doing ecstasy once probably isn't going to cause you much harm, but, and while it varies for the individual, if you take large amounts of it or take it too often or for too long you're definately pushing it.
Oh, and antidepressants don't generally help reverse the effect of this kind of damage either.
http://www.erowid.org has lots of good drug information (that's actually accurate).
Here is their page on ecstasy