 Early in the twentieth century Walter Cannon , who had popularized the idea of a <unk> @-@ adrenal system preparing the body for fight and flight , and his colleague Arturo <unk> developed a theory of two <unk> , sympathin E ( excitatory ) and sympathin I ( inhibitory ) , responsible for these actions . The Belgian pharmacologist <unk> <unk> as well as Canadian and US @-@ American pharmacologists between 1934 and 1938 suggested that noradrenaline might be a sympathetic transmitter . In 1939 , Hermann <unk> and Peter Holtz independently identified the biosynthetic mechanism for norepinephrine in the vertebrate body . In 1945 Ulf von Euler published the first of a series of papers that established the role of norepinephrine as a neurotransmitter . He demonstrated the presence of norepinephrine in sympathetically innervated tissues and brain , and adduced evidence that it is the sympathin of Cannon and <unk> . 
