There's an interesting book called "Why am I still depressed?" You can read a short summary/reaction here: Why am I still depressed? (Bipolar 11 and Soft Bipolar)

In this book, the author discusses in great detail the entire bipolar spectrum. He proposes a theory that some people don't experience hypomania or mania at all, but instead only experience depression and awful mixed states, where the awful mixed states are mistaken for severe depression. As a result, when a bipolar person who only experiences depression and mixed states is given an antidepressant, it flings them into hypomania or mania. It is the first time they have ever experienced a pure "up".

So tl;dr... the author discusses how he believes bipolar disorder can exist without the "ups", and uses that theory to explain why some "normal" people become hypo/manic from ADs. It's a great book.