Raul R.Gainetdinov, Amy R.Mohn, Laura M.Bohn, and Marc G.Caron*

Howard Hughes Medical Institute Laboratories, Departments of Cell Biology and Medicine, Duke University Medical Center, Durham, NC 27710

In the brain, dopamine exerts an important modulatory influence over behaviors such as emotion, cognition, and affect as well as mechanisms of reward and the control of locomotion. The dopamine transporter (DAT), which reuptakes the released neurotransmitter into presynaptic terminals, is a major determinant of the intensity and duration of the dopaminergic signal. Knockout mice lacking the dopamine transporter (DAT-KO mice) display marked changes in dopamine homeostasis that result in elevated dopaminergic tone and pronounced locomotor hyperactivity. A feature of DAT-KO mice is that their hyperactivity can be inhibited by psychostimulants and serotonergic drugs. The pharmacological effect of these drugs occurs without any observable changes in dopaminergic parameters, suggesting that other neurotransmitter systems in addition to dopamine might contribute to the control of locomotion in these mice. We report here that the hyperactivity of DAT-KO mice can be markedly further enhanced when N-methyl-D-aspartate receptor-mediated glutamatergic transmission is blocked. Conversely, drugs that enhance glutamatergic transmission, such as positive modulators of L-α-amino-3-hydroxy-5-methylisoxazole-4-propionate glutamate receptors, suppress the hyperactivity of DAT-KO mice. Interestingly, blockade of N- methyl-D-aspartate receptors prevented the inhibitory effects of both psychostimulant and serotonergic drugs on hyperactivity. These findings support the concept of a reciprocal functional interaction between dopamine and glutamate in the basal ganglia and suggest that agents modulating glutamatergic transmission may represent an approach to manage conditions associated with dopaminergic dysfunction.

Frontostriatal circuitry is one of the most prominent brain pathways involved in the control of locomotion, affect, impulsivity, attention, and emotion (1, 2). One axis of this circuitry involves dopaminergic projections into the striatal and mesolimbic brain areas (1, 3). Dopaminergic transmission has been intensively studied and is relatively well characterized (1, 3), largely because alterations in dopaminergic tone have clear behavioral manifestations such as changes in locomotor activity. In addition to dopaminergic innervation from substantia nigra and ventral tegmental area, the basal ganglia receive dense glutamatergic input predominantly from prefrontal cortical areas, as well as from the hippocampus, periventricular thalamus, and amygdala (1, 4, 5). There is a growing appreciation for the concept that dopaminergic and glutamatergic systems intimately interact at the level of medium-sized spiny neurons in the basal ganglia to control behavior (1, 6, 7). Particularly, an interaction at the levels of receptor signaling and regulation between dopamine D1 and/or D2-like receptors and ionotropic glutamate N-methyl-D-aspartate (NMDA) and L-α-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors, has been put forth (7–9). Recent findings in mice with decreased NMDA receptor expression (10) confirmed and extended previous pharmacological studies (1, 11, 12), suggesting a reciprocal interaction of glutamatergic and dopaminergic transmission in the control of motor behaviors. Several lines of evidence suggest that serotonin (5-HT) also plays an important role in this interaction (13–17), in part by modulating the activity of glutamatergic neurons in the frontal cortex (12–15, 17). Nevertheless, the exact nature of the interplay of these systems within compartments of frontostriatal circuitry is still poorly understood.

By deleting the gene encoding the dopamine transporter (DAT), a strain of mice lacking the mechanism to provide reuptake of extracellular DA has been developed (18). In the absence of DAT, important changes are observed in extracellular dopamine dynamics and presynaptic homeostasis of dopaminergic terminals in the striatum of these mice, suggesting a prominent role of transporter function in the maintenance of normal homeostatic control (19). Because of their characteristics, the DAT-knockout (DAT-KO) mice represent an interesting animal model in which the interplay between various neuronal systems can be examined.

Pharmacological evidence would predict that elimination of the DAT should result in changes in the extracellular dynamics of dopamine (3). Cyclic voltammetry experiments in mouse striatal slices demonstrated a 300-fold increase in the amount of time dopamine spends in the extracellular space (18–20). Moreover, cyclic voltammetry studies revealed that the rate of dopamine clearance was unaltered either by inhibitors of other transporters or by selective inhibitors of the dopamine degradation enzymes (19). Amphetamine, methylphenidate, and cocaine were found to be unable to affect clearance or extracellular dopamine levels in the striatum of DAT-KO mice (18, 19, 21). These observations suggested that over the time it takes to clear dopamine released by a single pulse stimulation, diffusion alone plays the major role in removing dopamine from the extracellular space in the striatum of DAT-KO mice (19). To directly prove that this prolonged clearance could result in alterations in extracellular dopamine concentrations, an alternative approach to assess extracellular dopamine dynamics, a quantitative “no net flux” microdialysis technique in freely moving mice was used (19, 20). These studies revealed a 5-fold elevation in steady-state striatal extracellular dopamine in DAT-KO mice in comparison to wild-type (WT) mice. These simple neurochemical parameters would suggest that the DAT-KO mice could represent a genetic model of persistent functional hyperdopaminergia.

From these initial neurochemical characterization, it became clear that in DAT-KO mice not only changes in extracellular dopamine dynamics but also numerous alterations in both presynaptic and postsynaptic components of dopaminergic trans-

Front Matter (R1-R5)

1 Introduction (10994-10995)

2 Neural Roles for Heme Oxygenase: Contrasts to Nitric Oxide Synthase (10996-11002)

3 Presynaptic Kainate Receptors at Hippocampal Mossy Fiber Synapses (11003-11008)

4 Retrograde Signaling at Central Synapses (11009-11015)

5 Controlling Potassium Channel Activities: Interplay Between the Membrane and Intracellular Factors (11016-11023)

6 Calcium Regulation of Neuronal Gene Expression (11024-11031)

7 Sensory Experience and Sensory Activity Regulate Chemosensory Receptor Gene Expression in Caenorhabditis Elegans (11032-11038)

8 Presenilin, Notch, and the Genesis and Treatment of Alzheimer's Disease (11039-11041)

9 FosB: A Sustained Molecular Switch for Addiction (11042-11046)

10 Glutabatergic Modulation of Hyperactivity in Mice Lacking the Dopamine Transporter (11047-11054)

11 Zinc Induces a Src Family Kinase-Medicated Up-Regulation of NMDA Receptor Activity and Excitotoxicity (11055-11061)

12 Regulation of Cyclin-Dependent Kinase 5 and Casein Kinase 1 by Metabotropic Glutamate Receptors (11062-11068)