Altered Reward Circuitry in the Norepinephrine Transporter Knockout Mouse

et al. (2013) Altered Reward Circuitry in the Norepinephrine Transporter Knockout Mouse. PLoS ONE 8(3): e57597. doi:10.1371/journal.pone.0057597 Altered Reward Circuitry in the Norepinephrine Transporter Knockout Mouse Joseph J. Gallagher 0 Xiaowei Zhang 0 F. Scott Hall 0 George R. Uhl 0 Elaine L. Bearer 0 Russell E. Jacobs 0 Eric M. Mintz, Kent State University, United States of America 0 1 Biological Imaging Center, Beckman Institute, California Institute of Technology, Pasadena, California, United States of America, 2 Department of Pathology, University of New Mexico Health Sciences Center , Albuquerque , New Mexico, United States of America , 3 Molecular Neurobiology Branch , National Institute on Drug Abuse, Intramural Research Program , Baltimore, Maryland , United States of America Synaptic levels of the monoamine neurotransmitters dopamine, serotonin, and norepinephrine are modulated by their respective plasma membrane transporters, albeit with a few exceptions. Monoamine transporters remove monoamines from the synaptic cleft and thus influence the degree and duration of signaling. Abnormal concentrations of these neuronal transmitters are implicated in a number of neurological and psychiatric disorders, including addiction, depression, and attention deficit/hyperactivity disorder. This work concentrates on the norepinephrine transporter (NET), using a battery of in vivo magnetic resonance imaging techniques and histological correlates to probe the effects of genetic deletion of the norepinephrine transporter on brain metabolism, anatomy and functional connectivity. MRS recorded in the striatum of NET knockout mice indicated a lower concentration of NAA that correlates with histological observations of subtle dysmorphisms in the striatum and internal capsule. As with DAT and SERT knockout mice, we detected minimal structural alterations in NET knockout mice by tensor-based morphometric analysis. In contrast, longitudinal imaging after stereotaxic prefrontal cortical injection of manganese, an established neuronal circuitry tracer, revealed that the reward circuit in the NET knockout mouse is biased toward anterior portions of the brain. This is similar to previous results observed for the dopamine transporter (DAT) knockout mouse, but dissimilar from work with serotonin transporter (SERT) knockout mice where Mn2+ tracings extended to more posterior structures than in wildtype animals. These observations correlate with behavioral studies indicating that SERT knockout mice display anxiety-like phenotypes, while NET knockouts and to a lesser extent DAT knockout mice display antidepressant-like phenotypic features. Thus, the mainly anterior activity detected with manganese-enhanced MRI in the DAT and NET knockout mice is likely indicative of more robust connectivity in the frontal portion of the reward circuit of the DAT and NET knockout mice compared to the SERT knockout mice. - Funding: The project was funded in part by the Beckman Institute, NIH NINDS NS062184, NIMH MH087660, NIMGS P5OGM08273 (ELB), and NIDA R01DA18184 (REJ) and, in part, by the National Institute on Drug Abuse, Intramural Research Program (GRU and FSH), and by the Harvey Family endowment (ELB). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. . These authors contributed equally to this work. Norepinephrine (NE) is a monoamine neurotransmitter implicated in various behavioral and psychological functions including learning and memory, anxiety, arousal, and mood; as well as disorders related to these processes (e.g. addiction, depression, attention deficit/hyperactivity disorder) [15]. NE innervation for much of the brain comes from cell bodies of the locus coeruleus (LC). These neurons have diffuse projections to many brain regions with particularly dense innervation in limbic regions, as well as the frontal cortex, and other monoaminergic nuclei (i.e. serotonergic raphe nuclei and dopaminergic ventral tegmental area). The norepinephrine transporter (NET, SLC6A2) is responsible for norepinephrine reuptake by the presynaptic terminal. Thus, it removes NE from the synaptic cleft and terminates noradrenergic neurotransmission, while re-charging presynaptic cells for future transmission. NET is a direct target of both antidepressants and psychostimulants [6,7]. Additionally, NET mediates dopamine uptake in the prefrontal cortex [810]. Recent work in animal models has suggested that the mechanism of drugs that treat ADHD may include inhibition of fronto-cortical NET [11,12]. NE and NET, along with two other monoamines and their transporters (DAT: dopamine transporter, SERT: serotonin transporter) form a complex interacting system that influences a broad range of affective states. Mouse knockouts for NET, DAT, and SERT have been used to study the pharmacological, behavioral, (...truncated)