Peripheral and central serotonergic systems are both relevant:
> Because 5-HT cannot cross the blood-brain barrier, the peripheral 5-HT system is functionally separate from the central 5-HT system. — Diabetes Metab J. 2016 https://pubmed.ncbi.nlm.nih.gov/27126880/
> Because 5-HT cannot cross the blood-brain barrier, the peripheral 5-HT system is functionally separate from the central 5-HT system. — Diabetes Metab J. 2016 https://pubmed.ncbi.nlm.nih.gov/27126880/