HTR3A and Erythema: Stimulation of MCs by NPs, such as 5-hydroxytryptamine (serotonin) receptor 3A (HTR3A), pituitary adenylate cyclase-activating polypeptide (PACAP), substance P (SP), and vasoactive intestinal peptide (VIP), can cause MC degranulation and the release of histamine, tryptase and other mediators (e.g., TNF-α, CXCL9, CXCL10, and CXCL8), thereby promoting inflammation and resulting in itching, flushing, erythema, and/or burning sensations, which are common symptoms of rosacea.