Depressants belong to a general class of drugs that decrease activity in one or more parts of the nervous system. Depression in the usual sense of the word comprises a particular subset of depressed states, particulars as it refers to lack of happiness or drive. Not all depressed states are considered depression in the common sense of the word. Similarly, not all stimulated states are the opposite of this depression. Different parts of the brain can be depressed or stimulated, and the affects of these effects come in many forms and resulting psychological states. Generally speaking, a depressant can be used to treat a condition if that particular depressant decreases activity in a part of the nervous system that is overactive in that condition. Anxiety, as you have mentioned, is a prime example condition where one part of the brain is overactive, for which certain depressant drugs can calm that part of the brain to effect relief from the anxiety. If a particular case of depression is caused by overactivity in one or more brain regions, then it could in theory be possible to treat that depression using depressants that decrease activity in those particular brain regions. The key thing to keep in mind here is that mental depression can be caused by over or underactivity in various brain regions, so there is a possibility of using either specific types of stimulants or specific types of depressants to effect relief.