What is the cause of depression and what part in the brain is causing it? ?

Scientists aren't too completely sure of that, but they do believe the answer to be that several parts of the nerves may be the cause of depression, or at least, have a major impact on the brain that might result to depression. As for a sure answer, I'm afraid that we'll have to wait for the researchers to look more into the cause of moods and other such subjects before drawing a final conclusion on the matter.

Olfactory epithelium i believe