A molecular "messenger" inside cells that relays instructions from surface receptors to the machinery inside the cell — a pivotal relay in many peptide signaling pathways.Produced by adenylyl cyclase when Gs-coupled receptors are activated. cAMP activates protein kinase A (PKA), which phosphorylates downstream targets including CREB (altering gene expression), MLCK (relaxing smooth muscle), and MITF (driving melanogenesis). This single second messenger is central to melanocortin, GLP-1, VIP, secretin, and many other peptide receptor pathways — explaining why selective receptor agonism matters so much when you want a specific effect without activating the whole cascade.