Transmission in noradrenergic neurons is somewhat more complex, particularly in regard to the mechanisms by which the transmitter is removed from the biophase subsequent to its release. Noradrenergic transmission is represented diagrammatically in Figure 9.3.
Synthesis of norepinephrine begins with the amino acid tyrosine, which enters the neuron by active transport, perhaps facilitated by a permease. In the neuronal cytosol, tyrosine is converted by the enzyme tyrosine hydroxylase to dihydroxyphenylalanine (dopa), which is converted to dopamine by the enzyme aromatic L-amino acid decarboxylase, sometimes termed dopa-decarboxylase. The dopamine is actively transported into storage vesicles, where it is converted to norepi-nephrine (the transmitter) by dopamine 3-hydroxylase, an enzyme within the storage vesicle.
In noradrenergic neurons, the end product is norepi-nephrine. In the adrenal medulla, the synthesis is carried one step further by the enzyme phenylethanolamine N-methyltransferase, which converts norepinephrine to epinephrine. The human adrenal medulla contains approximately four times as much epinephrine as norepi-nephrine. The absence of this enzyme in noradrenergic neurons accounts for the absence of significant amounts of epinephrine in noradrenergic neurons. The structures of these compounds are shown in Figure 9.4.
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