Neuronal function can be modulated by a variety of neuronal, environmental and hormonal stimuli. One form of neuronal modulation is the change in the biosynthesis of specific neurotransmitters. This is of particular interest since neurotransmitters are the agents responsible for neuronal communication. The analysis of the long-term modulation of neurotransmitter expression in response to external factors could be a suitable model to study the possible biochemical mechanisms involved in learning and memory. Furthermore, understanding the molecular mechanisms involved in the regulation of norepinephrine synthesis in the sympathetic nervous system may be relevant for understanding stress and diseases of the cardiovascular system.