You can be sure that when something is referred to as being neurotoxic, that it is; no matter how little you ingest. In other cases, enough rats have left their life to prove the "neurotoxicity at higher doses", and for establishing an LD50...
In the case of 4-Br-amphetamine, the neurotoxicity is apparent even at low doses, if I remember right - in human trials, several subjects experienced parkinson-like tremors over the course of the following week after ingesting several milligrams under medical supervision!
(ugly stuff IMO!)
EDIT: Apparently my memory didn't serve me right; it was several miligrams per kilo of body weight, not overall dose....
indole_amine