Hey, I know this is an ancient thread but this features prominently on google so I feel justified to link an article for future visitors!

. Some evidence suggests that serotonin receptor down-regulation is the actual cause of the efficacy of anti-depressants, because serotonin triggers the release of excitory neurotransmitters such as glutamate.
https://www.docdroid.net/qbj8/andrews2015.pdf.html .This does seem to explain why it takes SSRIs several weeks to work but of course this doesn't seem to explain the SSRI withdrawal symptoms, unless the disequilibrium caused by withdrawing just messes up other neurotransmitter pathways.