neuropeptide

A neuropeptide is a tiny protein-like molecule that brain cells use to send slow, lingering messages to one another and to the rest of the body. To picture it, think of two kinds of mail. The fast classic neurotransmitters are like a text message: a quick ping that arrives and fades in an instant. A neuropeptide is more like a handwritten letter — it takes longer to arrive, but its mood can hang in the air for minutes. Chemically, a neuropeptide is just a short chain of amino acids (the same building blocks that make up larger proteins), usually somewhere between about 3 and 40 links long, which is why scientists call it small.

These molecules sit at the crossroads of the nervous system and the hormone (endocrine) system, which is why they belong to the brain–body axis. A neuropeptide rarely flips a switch on its own; instead it tunes the dial, making nearby circuits more or less excitable and coloring how you feel. Familiar examples include oxytocin (tied to bonding and trust), endorphins (your built-in painkillers), and substance P (which carries pain signals). The same molecule can act locally between two neurons or travel through the blood like a hormone to reach distant organs, letting the brain shape hunger, stress, sleep, and emotion throughout the whole body.