Vasopressin

Also known as antidiuretic hormone, vasopressin is a potent endogenous hormone which is responsible for regulating plasma osmolality and volume. It acts as a neurotransmitter in the brain to control circadian rhythm, thermoregulation, and adrenocorticotrophic hormone release (ACTH).

Reference:
Sharman, A., & Low, J. (2008). Vasopressin and its role in critical care. Continuing Education in Anaesthesia Critical Care & Pain, 8(4), 134-137.