I'm speaking from anecdotal evidence here but I have seen a lot of research that indicate the digestive tract is basically the primary storage of serotonin. And whenever I get chronically depressed I also get chronically constipated. I don't necessarily think probiotics are a magical cure but I think anything that aids with digestion and moving things through the digestive tract can help.
> the digestive tract is basically the primary storage of serotonin.
What does this even mean? Monoamines aren't fat or glycogen stored in the body. The digestive tract has serotonin receptors, like the brain, but they differ in purpose entirely. Monamines are just messenger chemicals. Their context determines the actual message. I get it... it makes sense, humans are basically worms and worms are basically digestive tracts, that's our main thing, but taking this fact and making predictive science out of it is not trivial at all and anecdotal evidence means absolutely nothing so please never ever ever ever mention it in public forums.
Serotonin does not cross the blood brain barrier. Quite often neurotransmitters are used in different parts of the body and have different purposes.
Gut motility is driven by the autonomic nervous system. The sympathetic nervous system controls colon motility, and the parasympathetic controls digestion and stomach emptying. Depending on the type of depression you have, it can alter the HPA axis up or down. Let's say you have the "don't want to get out of bed and do anything" type. In that case your HPA axis is likely underactive. That results in reduced sympathetic nervous system activation (as it operates in parallel with the HPA axis). That then results in reduced colon motility and constipation.
Sometimes often I get constipation at weekends for this reason. I'm not depressed, but if I don't do much at the weekend after a busy week then (I assume) my HPA axis reduces, and that causes constipation.
