Acetylcholine

Acetylcholine was involved in the "chemical war" described by the Tenth Doctor, which was going on in Donna Noble's head as she walked down the aisle for her Christmas Eve 2007 wedding. He briefly hypothesised that the adrenaline, acetylcholine and endorphins had her particles reaching "boiling point", perhaps leading to her sudden appearance in the TARDIS. (TV: The Runaway Bride)