MDD and bipolar depression are sometimes identical, but sometimes bipolar depression can be more atypical. One of the psychiatrists I work with has a theory that MDD is a problem with serotonin, while bipolar depression is a lack of dopamine. He thinks we burn through our dopamine with mania and sort of use it up, then crash into depression when this feel good neurotransmitter is gone. It's his theory, and I have no idea if he's right, because I've not read or heard about this elsewhere. But it is an interesting idea.