MPTP which is the actual dangerous species which makes MPP+ in vivo, theoretically MPP+ cannot hurt even if you inject it unless you get it past the BBB not that I would want to test the theory, and the betacarbolnes are close cousins, if a nitrogen bridge is made between the piperidine and phenyl of MPTP you get a tetrahydrobetacarboline, and yes this betacarboline is an effective DA neurotoxin too. it is this 'derivative' of MPTP that shulgin refers to in the back of Pihkal. there are also some pretty effective isoquinoline neurotoxins which work the same way, by becoming permanently charged inside the the brain so they can't get back out and then they spin out free radicals as the oxidase system tries to chew them up
does B caapi contain tryptamines as well as carbolines? or am I thinking of some other South American plant.
My only harmine experience was with material extracted from Aldricha chimica and it was delerium. perhaps Rue or B caapi are kinder materials though rue contains some other unpleasant alkaloids.