People get confused about what 'depressant' means. It's a depressant insofar as it depresses your breathing and heart rate, not in that it makes you depressed. That's not to say opiate addicts can't also be depressed, but that's not what it means to be a depressant as a class of drugs.

With that said, I don't think opiates are a good treatment option for bipolar due to their addictive nature, tolerance, the risk of overdose, and the tendency for people with bipolar to be impulsive. With THAT said, opiates have made me very happy in the past when I did them recreationally in college...but it never lasted.