I don't have a citation to hand, only my recent discussions about this, from someone that should equally know what they are talking about:

The neurotoxicity has been demonstrated in rats, but at a dose that would be fatal in humans. Rats have better tolerance of the effect that mdma has on thermo-regulation and this allows the neurotoxic effect to manifest.

If you gave a dose to a human that was sufficient to kill nerve cells, you would kill the human.

Like I said, no citation to hand (someone else maybe??), but this is food for thought I think.