A neuropeptide is any of a family of molecules formed by a short chain of amino acids and which plays a role in humanoid brain activity. An increase or decrease in neuropeptide levels may be a cause or symptom of abnormal brain activity.
An overabundance of neuropeptides was the scientific explanation offered by Doctor Julian Bashir in 2373 for Captain Benjamin Sisko's Orb shadow hallucinations. These can occur a long time after a direct Bajoran Orb experience. Sisko accepted treatment to restore his neuropeptide levels and make the visions stop. (DS9: "Rapture")
When a Human is made part of a Borg neural link via a neural transceiver, its neuropeptide levels became abnormally high until the connection was severed. Chakotay experienced this in 2373, and The Doctor hypothesized that this increased his telepathic receptivity. (VOY: "Unity")
In 2373, Commander Chakotay noted that a piece of organic technology aboard a Species 8472 bio-ship looked like a binary matrix but it was laced with neuropeptides, which he further speculated to be its version of a computer core. (VOY: "Scorpion")
In 2378, Lieutenant Commander Tuvok required fal-tor-voh, a treatment for a Vulcan degenerative neurological disease, which involves receiving a mind meld. The disease causes the neuropeptide levels in the Vulcan brain to decrease, and affects their logic capabilities. (VOY: "Endgame")