Doesn't "depressant" have more to do with your body's functions and not necessarily the depression we think about? Been a long time since high school health and EMT classes.
I've heard that drugs, including alcohol, trigger the chemicals in our brains that make us feel "happy". After the effects wear off, our natural "happiness" level is even lower than it was before. This is why we feel the need to use the drug again, and over time, we also have to take more of it to feel the same level of "happiness".