Norepinephrine (noradrenaline) is produced from the amino acid precursor tyrosine. Tyrosine yields L-dopa by way of the enzyme tyrosine hydroxylase, which is the rate-limiting enzyme [7]. L-dopa is converted to dopamine by dopa decarboxylase. The enzyme dopamine ?-hydroxylase converts dopamine to norepinephrine. The noradrenergic cell groups in the brain are located in the locus ceruleus and the lateral tegmental nuclei of the brainstem [6,7,10]. Both project extensively, and the locus ceruleus, which probably plays a modulation and integrative role, is the most widely projecting nucleus in the CNS [6].

Norepinephrine neurotransmission is terminated by the action of the norepinephrine transporter protein. Norepinephrine is transported from the synaptic cleft back into the neuron. Adrenergic receptors include ? and ? groups with several subtypes (Table 3.3) [6,7,10,11]. Norepinephrine receptors belong to the G-protein family. These receptors are found in the CNS as well as peripherally [11].