 Inside the brain norepinephrine functions as a neurotransmitter , and is controlled by a set of mechanisms common to all monoamine neurotransmitters . After synthesis , norepinephrine is transported from the cytosol into synaptic vesicles by the vesicular monoamine transporter ( VMAT ) . Norepinephrine is stored in these vesicles until it is ejected into the synaptic cleft , typically after an action potential causes the vesicles to release their contents directly into the synaptic cleft through a process called exocytosis . 
