You might be thinking of mushroom spines a term term that's used to refer to fully mature spines that have an axon at their ends.
I've been doing some reading about this.
It was definitely psilocybin that I read about causing dendritic structures lost due to depression to regrow. I just accidently found an article about it searching for information about MDMA and dendritic structures. I think this is the whole reasoning behind microdosing actually.
Also, from what I've read it looks like meth, MDMA, and cocaine all cause somewhat similar (to each other) changes to dendritic structures in various brain regions with repeated exposure. They can each lower expression of certain dendritic structures in certain brain regions while increasing their expression in other regions.
I'm not sure that this is indicative of neurotoxicity or that it happens upon a single exposure. It looks more to me like a function of the brain remodeling itself to adapt to the presence of a stimulant (repeated exposure to that stimulant leading to a chronic abundance of neurotransmitters in the brain and the brain trying to adapt to accommodate this bombardment).
In the study I read regarding MDMA and dendritic structures the mice were injected twice daily with 5mg/kg of MDMA, every day for 3 weeks in order to induce changes. One of the stated objectives of the article was to find out whether or not MDMA actually produced these changes, and the article also stated at the beginning that cocaine and meth produced similar changes.
I recently read that some European harm reduction agency recommended humans not to exceed 1.5mg/kg (150mg for a 100kg person) so those mice were rolling their asses off twice a day for three weeks solid in order to produce these changes.
Taking all of that into account I think maybe you're assessment that MDMA is more neurotoxic than meth based on changes in dendritic structures may be false. Especially since I read another article about meth causing similar changes in the expression of dendritic structures throughout the brain.
Keep in mind that I'm not arguing that MDMA is not capable of causing more long lasting changes to serotonergic structures in the brain, only that mg for mg methamphetamine is more toxic overall (not just to the brain but other parts of the body as well).
Why I'm arguing that I dunno. Meth in my system encouraging me to delve into the subject most likely!!
Cheers!