@cwis- Serotonin is one of many hormones that controls your appetite.
https://www.psychologytoday.com/us/blog/the-antidepressant-diet/201007/reversing-antidepressant-weight-gain
https://www.mayoclinic.org/diseases-conditions/depression/expert-answers/antidepressants-and-weight-gain/faq-20058127
There are a lot of reasons that weight gain can take effect. When we are depressed some of us tend to eat more, move less. It's very common for me to say, "Oh what the hell" and eat everything in sight. The I get more depressed because I've gained weight. Also as we grow older our bodies change and we tend, especially after menopause, to gain weight because we have lost most of our estrogen.
So far there is not one cause yet accepted for weight gain while taking antidepressants. There are many theories but since the brain is so complicated it's yet a mystery.
Here's another site to help. Feel free to google more and please share with us your findings!