The autonomic nervous system is part of the central nervous system. It mainly controls viseral function. The autonomic nervous system is made up of two types of nerves, the parasympathetic nerves and the sympathetic nerves. The neurotransmitter of parasympathetic nerves is acetylcholine. There is no parasympathetic innervation in the mammary gland.
Norepinephrine is a neurotransmitter from peripheral nerves and nerves in the brain.
Norepinephrine also can from the adrenal medulla. There are pressure-sensitive neurons around the cisterns and the large ducts. If you electrically stimulate the cut end of the mammary nerves, you do not get milk ejection. Norepinephrine and epinephrine can inhibit oxytocin-induced contraction of myoepithelial cells. Stressful stimuli will inhibit milk ejection.