Dopamine and Serotonin Systems: how are they related?

[Felix Guattari]

Perhaps other SWiMers will be able to help me hash out a constellation of questions relating to general pharmacology that I've been gnawing at for a while. I've tried to research these to the best of my ability/resources and though I suspect there are answers (or at least relevant research) to these questions, it may be the case that there aren't, in which case guesses would be just as good.

1) It is often said that raised dopamine levels will eventually suppress serotonin levels. Is it known by what mechanism?

2) Does anyone else know of cases (or can confirm their theoretical possibility) wherein long-term SSRI use has depressed dopamine to pathologically low levels?