A neurotransmitter in the Betazoid brain that makes telepathy possible, the depletion of which contributed to Lwaxana Troi's collapse while she was teaching the Cairn to speak in Dark Page.
Psilosynine is a chemical messenger found in the brains of telepathic species such as Betazoids. It is active in the paracortex, the region responsible for telepathy, and heavy telepathic effort uses it up faster than the body can replace it.
The substance is discussed in the Next Generation episode Dark Page. Lwaxana Troi has spent months acting as interpreter and tutor to the Cairn, a people who communicate entirely by projected images and are only now learning spoken language. Doctor Crusher finds her psilosynine levels dangerously low and orders her to stop using her telepathy.
Lwaxana ignores the advice and falls into a coma. The exhaustion is only part of the cause: the strain has broken down a mental barrier she had built around the memory of her first daughter, Kestra, who drowned as a child. Deanna enters her mother's mind with the help of the Cairn Maques to bring her back.