Uh, reading the study, smoking cannabis then taking amphetamine causes dopamine deficits, and they don't explore whether this is transient or permanent.

I smoke more weed than I probably should, but I had a screwed up dopamine system to start with - the genes don't lie.

Actually, on that note, perhaps people with preexisting dopamine deficits are more likely to become heavy cannabis users because it's the only way that they can experience dopamine - going through life with people going "what an achievement! Aren't you proud??" and only being able to reply "meh, I should've done better than first place" or whatever sucks. I can only reflect upon and feel good about what I've done with my life when stoned. I don't have a reward cycle, otherwise - I learned to motivate myself with a punishment cycle instead, and only now am I slowly unlearning it.