The mice showed a ~20% loss at high dose. At the low dose, the mice showed no significant loss but were vulnerable to a sub-threshold dose of a toxin and toxin

The mice showed a ~20% loss at high dose. At the low dose, the mice showed no significant loss but were vulnerable to a sub-threshold dose of a toxin and toxin+MPH mice showed a ~20% loss. This is in figure 1, and in the discussion. Exactly what behavioral effects result from a 20% loss of DA neurons is still an open question. So, the clinical implication is that people on methylphenidate might be vulnerable to things (rotenone, heavy metals, etc) which aren't toxic to "normal" people.