INS and type 2 diabetes mellitus: In our working model, T2DM patients have high expression levels of 5-HT2CR in pancreatic β-cells, thus, after meal ingestion the intrapancreatic serotonergic nerves secrete 5-HT around pancreatic islets, which could activate the 5-HT2CR of pancreatic β-cells, resulting in decreased insulin secretion.