a neurotransmitter found mainly in areas of the brain that are involved in governing autonomic nervous system activity, especially blood pressure and heart rate.
Biology Dictionary
Definition:
A chemical that is both a hormone produced and secreted by the adrenal glands and a neurotransmitter, or substance which transmits nerve signals from neuron to neuron. As a hormone, it stimulates the storage or mobilization of sugar by acting on muscle and liver cells.
WordNet Dictionary