A neurotoxin was a toxin which specifically affected the nervous system.
In 2151, a Klingon ship raided a Xarantine outpost and stole, among other things, Xarantine ale. The Xarantines, however, had bonded a carbon dioxide-based neurotoxin to a unique molecule of the ale. After the Klingons had consumed the ale, they became unsconscious and would have died within 1 or 2 days, but Phlox developed an antidote. (ENT: "Sleeping Dogs")
A substance produced by Xindi-Insectoid eggs was a mild neurotoxin to the Human nervous system. Jonathan Archer was contaminated with this neurotoxin in 2153; in addition, this substance also subconsciously forced him to care for the eggs like a Xindi-Insectoid. (ENT: "Hatchery")
In 2371, the Kazon used a neurotoxin to kill one of their own who was gravely injured while attempting to integrate a Federation replicator into a Kazon vessel. It was injected through the use of a needle projecting from a ring. (VOY: "State of Flux")