Serotonin does not cross the blood brain barrier. Quite often neurotransmitters are used in different parts of the body and have different purposes. Gut motili

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.