Psilosynine was a neurotransmitter present in the paracortex of the brains of Betazoids and other species, which was connected with telepathic ability.
In 2370, Lwaxana Troi suffered from lowered psilosynine levels following extended telepathic contact with the Cairn. (TNG episode: "Dark Page") Later that year, a telepathic imprint in the structure of the USS Enterprise-D caused heightened levels of psilosynine in Counselor Deanna Troi. (TNG episode: "Eye of the Beholder")
In 2376, Crewman Soloman experienced heightened psilosynine levels after connecting to a telepathic Bynar pair in an alternate universe. (SCE eBook: Lost Time)