In the central nervous system, the neurotransmitter norepinephrine is involved in normal physiology, neuropsychiatric disorders, and the effects of numerous drugs. Although alterations of the central noradrenergic system are involved in the pathophysiology and pharmacotherapy of mood disorders, the basis and nature of these changes remain unresolved. Positron emission tomography (PET) and single photon emission computed tomography (SPECT) imaging agents will be valuable for further elucidating the roles of norepinephrine in health and disease. This review discusses PET and SPECT radioligands that have been developed for the enzymes, receptors, and transporters involved in noradrenergic neurotransmission. Currently, imaging agents that exhibit specific in vivo uptake in the brain have been described for monoamine oxidase A and β-adrenergic receptors, but have not undergone detailed evaluation or experimental application. Based on the successful development and utilization of in vivo imaging agents for elements of the central dopaminergic and serotoninergic systems, PET and SPECT radioligands are expected to serve as new tools for studying the physiology, pathophysiology, and pharmacology of the central noradrenergic system.

In recent years, a number of antidepressants with varying degrees of selectivity for the noradrenergic neurotransmitter system have become available. However, these agents represent a pharmacologically heterogeneous group and differ in terms of their precise side-effect profile and, possibly, their clinical efficacy. Bupropion, which is thought to act on both the dopamine and norepinephrine (NE) systems, has not been widely used as an antidepressant and has more recently been licensed as adjunctive therapy for smoking cessation. The serotonin-NE reuptake inhibitors venlafaxine and nefazodone, the noradrenergic and specific serotonergic antide-pressant mirtazapine, and the selective NE reuptake inhibitor reboxetine (the only truly NE-selective agent available) have all demonstrated efficacy in the treatment of depressive disorders. Evidence is now emerging for their use in the treatment of anxiety and panic disorders. There is some suggestion of a role for noradrenergic agents in other disorders, including attention-deficit/hyper-activity disorder and social phobia. The full range of disorders for which noradrenergic agents can be used remains to be seen and further research in this area is necessary.