Link to abstract:http://www.nature.com/mp/journal/vaop/ncurrent/full/mp201621...

The abstract doesn't really state if they attempted to test the effect of cannabis on dopamine with ongoning chronic use or after abstinence. It seems to be the latter, though, since the users stayed abstinent for 5-7 days before the brain scans. That's a rater short time for the brain to "recover" from chronic cannabis use.

It is also worth nothing that they measured dopamine binding by administrating amphetamine.