HTR5A and diabetic kidney disease: Studies have shown that the serum concentration of 5-HT in diabetic patients is increased, and the pathogenesis of diabetic nephropathy involves 5-HT2A receptor (5-HT2A) on renal mesangial cells (Kobayashi et al., 2008), which may be related to the production enhancement of type IV collagen by human mesangial cells induced by 5-HT in the kidney (Kasho et al., 1998).