Multiple lines of evidence demonstrate that the noradrenergic system provides both direct and indirect excitatory drive onto midbrain dopamine (DA) neurons. We used DA b-hydroxylase (DBH) knockout (DbhÀ/À) mice that lack norepinephrine (NE) to determine the consequences of chronic NE deficiency on midbrain DA neuron function in vivo. Basal extracellular DA levels were significantly attenuated in the nucleus accumbens (NAc) and caudate putamen (CP), but not prefrontal cortex (PFC), of DbhÀ/À mice, while amphetamine-induced DA release was absent in the NAc and attenuated in the CP and PFC. The decrease in dopaminergic tone was associated with a profound increase in the density of high-affinity state D 1 and D 2 DA receptors in the NAc and CP, while DA receptors in the PFC were relatively unaffected. As a behavioral consequence of these neurochemical changes, DbhÀ/À mice were hypersensitive to the psychomotor, rewarding, and aversive effects of cocaine, as measured by locomotor activity and conditioned place preference. Antagonists of DA, but not 5-HT, receptors attenuated the locomotor hypersensitivity to cocaine in DbhÀ/À mice. As DBH activity in humans is genetically controlled and the DBH inhibitor disulfiram has shown promise as a pharmacotherapy for cocaine dependence, these results have implications for the influence of genetic and pharmacological DBH inhibition on DA system function and drug addiction.

Although Parkinson's disease (PD) is characterized primarily by loss of nigrostriatal dopaminergic neurons, there is a concomitant loss of norepinephrine (NE) neurons in the locus coeruleus. Dopaminergic lesions induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) are commonly used to model PD, and although MPTP effectively mimics the dopaminergic neuropathology of PD in mice, it fails to produce PD-like motor deficits. We hypothesized that MPTP is unable to recapitulate the motor abnormalities of PD either because the behavioral paradigms used to measure coordinated behavior in mice are not sensitive enough or because MPTP in the absence of NE loss is insufficient to impair motor control. We tested both possibilities by developing a battery of coordinated movement tests and examining motor deficits in dopamine ␤-hydroxylase knockout (Dbh؊/؊) mice that lack NE altogether. We detected no motor abnormalities in MPTP-treated control mice, despite an 80% loss of striatal dopamine (DA) terminals. Dbh؊/؊ mice, on the other hand, were impaired in most tests and also displayed spontaneous dyskinesias, despite their normal striatal DA content. A subset of these impairments was recapitulated in control mice with 80% NE lesions and reversed in Dbh؊/؊ mice, either by restoration of NE or treatment with a DA agonist. MPTP did not exacerbate baseline motor deficits in Dbh؊/؊ mice. Finally, striatal levels of phospho-ERK-1/2 and ⌬FosB/FosB, proteins which are associated with PD and dyskinesias, were elevated in Dbh؊/؊ mice. These results suggest that loss of locus coeruleus neurons contributes to motor dysfunction in PD.dopamine ͉ Parkinson's disease ͉ dyskinesias ͉ dopamine ␤-hydroxylase P arkinson's disease (PD) affects Ϸ1% of the world's aging population (1). Despite this high prevalence and intensive research into its origins, the etiology of PD remains largely unknown. The disease is characterized by degeneration of dopamine (DA) neurons in the substantia nigra pars compacta (SN), and symptoms, which tend to manifest when Ϸ80% of striatal DA is lost, include bradykinesia, postural instability, rigidity, and resting tremor. 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a neurotoxin, is known to produce parkinsonism in humans and causes severe DA loss in animals (2). Because of its ability to recapitulate the neuropathology of PD, MPTP is used widely in PD research. However, MPTP has been unable to reliably reproduce the motor symptoms of PD in mice, which limits the utility of MPTP-treated mice as an animal model of the disease (3). Differences in mouse strain and experimental paradigms may at least partially account for these inconsistencies.Despite the focus on DA, PD is more accurately described as a multisystem disorder that features a profound albeit underappreciated loss of locus coeruleus (LC) neurons, as well as variable damage to other brain regions (4-6). Postmortem studies indicate that neuronal degeneration in the LC is comparable to that in the substantia nigra pars compacta, and that it may a...

The brainstem locus coeruleus (LC) supplies norepinephrine to the forebrain and degenerates in Alzheimer's disease (AD). Loss of LC neurons is correlated with increased severity of other AD hallmarks, including β-amyloid (Aβ) plaques, tau neurofibrillary tangles, and cognitive deficits, suggesting that it contributes to the disease progression. Lesions of the LC in amyloid-based transgenic mouse models of AD exacerbate Aβ pathology, neuroinflammation, and cognitive deficits, but it is unknown how the loss of LC neurons affects tau-mediated pathology or behavioral abnormalities. Here we investigate the impact of LC degeneration in a mouse model of tauopathy by lesioning the LC of male and female P301S tau transgenic mice with the neurotoxin N-(2-chloroethyl)ethyl-bromobenzylamine (DSP-4) starting at 2 months of age. By 6 months, deficits in hippocampal-dependent spatial (Morris water maze) and associative (contextual fear conditioning) memory were observed in lesioned P301S mice while performance remained intact in all other genotype and treatment groups, indicating that tau and LC degeneration act synergistically to impair cognition. By 10 months, the hippocampal neuroinflammation and neurodegeneration typically observed in unlesioned P301S mice were exacerbated by DSP-4, and mortality was also accelerated. These DSP-4-induced changes were accompanied by only a mild aggravation of tau pathology, suggesting that increased tau burden cannot fully account for the effects of LC degeneration. Combined, these experiments demonstrate that loss of LC noradrenergic neurons exacerbates multiple phenotypes caused by pathogenic tau, and provides complementary data to highlight the dual role LC degeneration has on both tau and Aβ pathologies in AD. Elucidating the mechanisms underlying AD is crucial to developing effective diagnostics and therapeutics. The degeneration of the LC and loss of noradrenergic transmission have been recognized as ubiquitous events in AD pathology, and previous studies demonstrated that LC lesions exacerbate pathology and cognitive deficits in amyloid-based mouse models. Here, we reveal a complementary role of LC degeneration on tau-mediated aspects of the disease by using selective lesions of the LC and the noradrenergic system to demonstrate an exacerbation of cognitive deficits, neuroinflammation, neurodegeneration in a transgenic mouse model of tauopathy. Our data support an integral role for the LC in modulating the severity of both canonical AD-associated pathologies, as well as the detrimental consequences of LC degeneration during disease progression.

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The data reveal potentially important and apparently additive effects of Dbh genotype and disulfiram administration on PFC catecholamine metabolism. These effects may have implications for genetic control of DBH activity in humans and for understanding therapeutic effects of disulfiram.