Figure 1 depicts the synthesis of dopamine and norepinephrine from the non-essential amino acid tyrosine. Within the central nervous system (CNS), dopamine and norepinephrine act as neurotransmitters; the cell groups producing catecholamines are localized in discrete brain regions and project their axon terminals to a wide range of target areas that play important roles in CNS functions (Kobayashi, 2001). Dopaminergic cells are mainly localized in the substantia nigra (A9 cell group) and ventral tegmental area (A10 cell group) (Lindvall et al., 1983). Neurons from the A9 group innervate the caudate-putamen to form the nigrostriatal pathway, which has a pivotal role in motor control (Gerfen,1992). Neurons from the A10 group project their fibers to the nucleus accumbens, amygdala, and prefrontal cortex, forming the mesocorticolimbic pathway, which is involved in emotion, motivation, and memory formation (Le Moal and Simon, 1991). Dopaminergic neurons from the A11-A14 groups are present in the mediobasal region of the hypothalamus, projecting their fibers to the median eminence and pituitary gland (Moore and Lookingland, 1995), modulating pituitary gland function. Farina et al.