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FIGURE 1–5. The noradrenergic system.This figure depicts the noradrenergic projections throughout the brain (A) and the various regulatory processes involved in norepinephrine (NE) neurotransmission (B). NE neurons innervate nearly all parts of the neuroaxis, with neurons in the locus coeruleus being responsible for most of the NE in the brain (90% of NE in the forebrain and 70% of total NE in the brain). The amino acid l-tyrosine is actively transported into presynaptic NE nerve terminals, where it is ultimately converted into NE. The rate-limiting step is conversion of l-tyrosine to l-dihydroxyphenylalanine (l-dopa) by the enzyme tyrosine hydroxylase (TH). -Methyl-p-tyrosine (AMPT) is a competitive inhibitor of tyrosine hydroxylase and has been used to assess the impact of reduced catecholaminergic function in clinical studies. Aromatic amino acid decarboxylase (AADC) converts l-dopa to dopamine (DA). l-dopa then becomes decarboxylated by decarboxylase to form dopamine (DA). DA is then taken up from the cytoplasm into vesicles, by vesicle monoamine transporters (VMATs), and hydroxylated by dopamine -hydroxylase (DBH) in the presence of O2 and ascorbate to form NE. Normetanephrine (NM), which is formed by the action of COMT (catechol-O-methyltransferase) on NE, can be further metabolized by monoamine oxidase (MAO) and aldehyde reductase to 3-methoxy-4-hydroxyphenylglycol (MHPG). Reserpine causes a depletion of NE in vesicles by interfering with uptake and storage mechanisms (depressive-like symptoms have been reported with this hypertension). Once released from the presynaptic terminal, NE can interact with a variety of presynaptic and postsynaptic receptors. Presynaptic regulation of NE neuron firing activity and release occurs through somatodendritic (not shown) and nerve-terminal 2 adrenoreceptors, respectively. Yohimbine potentiates NE neuronal firing and NE release by blocking these 2 adrenoreceptors, thereby disinhibiting these neurons from a negative feedback influence. Conversely, clonidine attenuates NE neuron firing and release by activating these receptors. Idazoxan is a relatively selective 2 adrenoreceptor antagonist primarily used for pharmacological purposes. The binding of NE to G protein receptors (Go, Gi, etc.) that are coupled to adenylyl cyclase (AC) and phospholipase C– (PLC-b) produces a cascade of second-messenger and cellular effects (see diagram and later sections of the text). NE has its action terminated in the synapse by rapidly being taken back into the presynaptic neuron via NE transporters (NETs). Once inside the neuron, it can either be repackaged into vesicles for reuse or undergo enzymatic degradation. The selective NE reuptake inhibitor and antidepressant reboxetine and older-generation tricyclic antidepressant desipramine are able to interfere/block the reuptake of NE. On the other hand, amphetamine is able to facilitate NE release by altering NET function. Green spheres represent DA neurotransmitters; blue spheres represent NE neurotransmitters. DAG = diacylglycerol; IP3 = inositol-1,4,5-triphosphate.Source. Adapted from Cooper JR, Bloom FE, Roth RH: The Biochemical Basis of Neuropharmacology, 7th Edition. New York, Oxford University Press, 2001. Copyright 1970, 1974, 1978, 1982, 1986, 1991, 1996, 2001 by Oxford University Press, Inc. Used by permission of Oxford University Press, Inc. Modified from Nestler et al. 2001.

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