Behavioral Phenotyping of Dopamine Transporter Knockout Rats: Compulsive Traits, Motor Stereotypies, and Anhedonia

Alterations in dopamine neurotransmission are generally associated with diseases such as attention-deficit/hyperactivity disorder (ADHD) and obsessive-compulsive disorder (OCD). Such diseases typically feature poor decision making and lack of control on executive functions and have been studied through the years using many animal models. Dopamine transporter (DAT) knockout (KO) and heterozygous (HET) mice, in particular, have been widely used to study ADHD. Recently, a strain of DAT KO rats has been developed (1). Here, we provide a phenotypic characterization of reward sensitivity and compulsive choice by adult rats born from DAT-HET dams bred with DAT-HET males, in order to further validate DAT KO rats as an animal model for preclinical research. We first tested DAT KO rats' sensitivity to rewarding stimuli, provided by highly appetitive food or sweet water; then, we tested their choice behavior with an Intolerance-to-Delay Task (IDT). During these tests, DAT KO rats appeared less sensitive to rewarding stimuli than wild-type (WT) and HET rats: they also showed a prominent hyperactive behavior with a rigid choice pattern and a wide number of compulsive stereotypies. Moreover, during the IDT, we tested the effects of amphetamine (AMPH) and RO-5203648, a trace amine-associated receptor 1 (TAAR1) partial agonist. AMPH accentuated impulsive behaviors in WT and HET rats, while it had no effect in DAT KO rats. Finally, we measured the levels of tyrosine hydroxylase, dopamine receptor 2 (D2), serotonin transporter, and TAAR1 mRNA transcripts in samples of ventral striatum, finding no significant differences between WT and KO genotypes. Throughout this study, DAT KO rats showed alterations in decision-making processes and in motivational states, as well as prominent motor and oral stereotypies: more studies are warranted to fully characterize and efficiently use them in preclinical research

Mice with Reduced NMDA Receptor Expression Display Behaviors Related to Schizophrenia

AbstractN-methyl-D-aspartate receptors (NMDARs) represent a subclass of glutamate receptors that play a critical role in neuronal development and physiology. We report here the generation of mice expressing only 5% of normal levels of the essential NMDAR1 (NR1) subunit. Unlike NR1 null mice, these mice survive to adulthood and display behavioral abnormalities, including increased motor activity and stereotypy and deficits in social and sexual interactions. These behavioral alterations are similar to those observed in pharmacologically induced animal models of schizophrenia and can be ameliorated by treatment with haloperidol or clozapine, antipsychotic drugs that antagonize dopaminergic and serotonergic receptors. These findings support a model in which reduced NMDA receptor activity results in schizophrenic-like behavior and reveals how pharmacological manipulation of monoaminergic pathways can affect this phenotype

Beyond cAMP: The Regulation of Akt and GSK3 by Dopamine Receptors

Brain dopamine receptors have been preferred targets for numerous pharmacological compounds developed for the treatment of various neuropsychiatric disorders. Recent discovery that D2 dopamine receptors, in addition to cAMP pathways, can engage also in Akt/GSK3 signaling cascade provided a new framework to understand intracellular signaling mechanisms involved in dopamine-related behaviors and pathologies. Here we review a recent progress in understanding the role of Akt, GSK3, and related signaling molecules in dopamine receptor signaling and functions. Particularly, we focus on the molecular mechanisms involved, interacting partners, role of these signaling events in the action of antipsychotics, psychostimulants, and antidepressants as well as involvement in pathophysiology of schizophrenia, bipolar disorder, and Parkinson’s disease. Further understanding of the role of Akt/GSK3 signaling in dopamine receptor functions could provide novel targets for pharmacological interventions in dopamine-related disorders

Activation of the spinal and brainstem locomotor networks during free treadmill stepping in rats lacking dopamine transporter

Dopamine is extremely important for the multiple functions of the brain and spinal cord including locomotor behavior. Extracellular dopamine levels are controlled by the membrane dopamine transporter (DAT), and animals lacking DAT (DAT-KO) are characterized by hyperdopaminergia and several alterations of locomotion including hyperactivity. Neuronal mechanisms of such altered locomotor behavior are still not fully understood. We believe that in hyperdopaminergic animals both the spinal and brain neuronal networks involved in locomotion are modified. Using the c-fos technique, we studied activated neuronal networks of the spinal cord and two brainstem structures related to locomotor control and being under the strong dopaminergic influence, the cuneiform nucleus (CnF) and ventrolateral periaqueductal gray (VLPAG), in wild-type (DAT-WT) and DAT-KO rats. In the spinal cord, most c-fos-positive cells were located in the dorsal laminae II-IV and in the central gray matter (laminae V-VI). No differences were revealed for the central areas. As for the dorsal areas, in the DAT-WT group, labeled cells mostly occupied the lateral region, whereas, in the DAT-KO group, c-fos-positive cells were observed in both medial and lateral regions in some animals or in the medial regions in some animals. In the brainstem of the DAT-WT group, approximately the same number of labeled cells were found in the CnF and VLPAG, but in the DAT-KO group, the VLPAG contained a significantly smaller number of c-fos-positive cells compared to the CnF. Thereby, our work indicates an imbalance in the sensorimotor networks located within the dorsal horns of the spinal cord as well as a disbalance in the activity of brainstem networks in the DAT-deficient animals

Assessment of DDAH1 and DDAH2 Contributions to Psychiatric Disorders via In Silico Methods

The contribution of nitric oxide synthases (NOSs) to the pathophysiology of several neuropsychiatric disorders is recognized, but the role of their regulators, dimethylarginine dimethylaminohydrolases (DDAHs), is less understood. This study’s objective was to estimate DDAH1 and DDAH2 associations with biological processes implicated in major psychiatric disorders using publicly accessible expression databases. Since co-expressed genes are more likely to be involved in the same biologic processes, we investigated co-expression patterns with DDAH1 and DDAH2 in the dorsolateral prefrontal cortex in psychiatric patients and control subjects. There were no significant differences in DDAH1 and DDAH2 expression levels in schizophrenia or bipolar disorder patients compared to controls. Meanwhile, the data suggest that in patients, DDAH1 and DDHA2 undergo a functional shift mirrored in changes in co-expressed gene patterns. This disarrangement appears in the loss of expression level correlations between DDAH1 or DDAH2 and genes associated with psychiatric disorders and reduced functional similarity of DDAH1 or DDAH2 co-expressed genes in the patient groups. Our findings evidence the possible involvement of DDAH1 and DDAH2 in neuropsychiatric disorder development, but the underlying mechanisms need experimental validation

Cross-hemispheric dopamine projections have functional significance

Decades of research have described dopamine’s importance in reward-seeking behavior and motor control. Although numerous investigations have focused on dopamine’s mechanisms in modulating behavior, the long-standing belief that dopamine neurons project solely unilaterally has limited the exploration of interhemispheric dopamine signaling. Here we resolve disparate descriptions of unilateral vs. bilateral projections by reporting that dopamine neurons can release dopamine in the contralateral hemisphere. Using voltammetry in awake and anesthetized rats, we reveal an unprecedented synchrony of dopamine fluctuations between hemispheres. Via stimulation with amphetamine, we demonstrate functional cross-hemispheric projections in a hemiparkinsonian model. This previously undescribed capacity for interhemispheric dopamine signaling can precipitate new areas of inquiry. Future work may exploit properties of bilateral dopamine release to improve treatments for Parkinson’s disease, including deep brain stimulation

Implanted Reuptake-deficient or Wild-type Dopaminergic Neurons Improve ON L-dopa Dyskinesias Without OFF-dyskinesias in a Rat Model of Parkinson's Disease

OFF-L-dopa dyskinesias have been a surprising side-effect of intrastriatal foetal ventral mesencephalic transplantation in patients with Parkinson's disease. It has been proposed that excessive and unregulated dopaminergic stimulation of host post-synaptic striatal neurons by the grafts could be responsible for these dyskinesias. To address this issue we transplanted foetal dopaminergic neurons from mice lacking the dopamine transporter (DATKO) or from wild-type mice, into a rat model of Parkinson's disease and L-dopa-induced dyskinesias. Both wild-type and DATKO grafts reinnervated the host striatum to a similar extent, but DATKO grafts produced a greater and more diffuse increase in extra-cellular striatal dopamine levels. Interestingly, grafts containing wild-type dopaminergic neurons improved parkinsonian signs to a similar extent as DATKO grafts, but provided a more complete reduction of L-dopa induced dyskinesias. Neither DATKO nor wild-type grafts induced OFF-L-dopa dyskinesias. Behavioural and receptor autoradiography analyses demonstrated that DATKO grafts induced a greater normalization of striatal dopaminergic receptor supersensitivity than wild-type grafts. Both graft types induced a similar downregulation and normalization of PEnk and fosb/Δfosb in striatal neurons. In summary, DATKO grafts causing high and diffuse extra-cellular dompamine levels do not per se alter graft-induced recovery or produce OFF-L-dopa dyskinesias. Wild-type dopaminergic neurons appear to be the most effective neuronal type to restore function and reduce L-dopa-induced dyskinesias

TAAR1-dependent effects of apomorphine in mice.

G protein-coupled trace amine-associated receptor 1 (TAAR1) is expressed in several brain regions and modulates dopaminergic activity partially by affecting D2 dopamine receptor function. In vitro , the nonselective dopamine agonist apomorphine can activate mouse and rat TAAR1. The aim of the present study was to evaluate whether apomorphine activity at the rodent TAAR1 observed in in vitro studies contributes to its behavioral manifestation in mice. For this purpose, we compared the behavioral effects of a wide range of apomorphine doses in wild type (WT) and TAAR1 knockout (TAAR1-KO) mice. Apomorphine-induced locomotor responses (0.01–4.0 mg/kg) were tested in locomotor activity boxes, and stereotypic behavior at 5 mg/kg was tested by ethological methods. A gnawing test was used to analyze the effects of the highest dose of apomorphine (10 mg/kg). No statistically significant differences were observed between TAAR1-KO and WT mice following inhibitory pre-synaptic low doses of apomorphine. At higher doses (2.0–5.0 mg/kg), apomorphine-induced climbing behavior was significantly reduced in TAAR1 mutants relative to WT controls. Moreover, the lack of TAAR1 receptors decreased certain types of stereotypies (as reflected in by measures of the global stereotypy score, licking but not sniffing or gnawing) that were induced by high doses of apomorphine. These data indicate that apomorphine activity at TAAR1 contributes to some behavioral manifestations, particularly climbing, in rodents following high doses of this drug. The contribution of TAAR1 to apomorphine-induced climbing in rodents should be considered when apomorphine is used as a screening tool in the search for potential antipsychotics

Improved cognitive performance in trace amine-associated receptor 5 (TAAR5) knock-out mice

Trace amine-associated receptors (TAARs) are a family of G protein-coupled receptors present in mammals in the brain and several peripheral organs. Apart from its olfactory role, TAAR5 is expressed in the major limbic brain areas and regulates brain serotonin functions and emotional behaviours. However, most of its functions remain undiscovered. Given the role of serotonin and limbic regions in some aspects of cognition, we used a temporal decision-making task to unveil a possible role of TAAR5 in cognitive processes. We found that TAAR5 knock-out mice showed a generally better performance due to a reduced number of errors and displayed a greater rate of improvement at the task than WT littermates. However, task-related parameters, such as time accuracy and uncertainty have not changed significantly. Overall, we show that TAAR5 modulates specific domains of cognition, highlighting a new role in brain physiology

Altered Neurocircuitry in the Dopamine Transporter Knockout Mouse Brain

The plasma membrane transporters for the monoamine neurotransmitters dopamine, serotonin, and norepinephrine modulate the dynamics of these monoamine neurotransmitters. Thus, activity of these transporters has significant consequences for monoamine activity throughout the brain and for a number of neurological and psychiatric disorders. Gene knockout (KO) mice that reduce or eliminate expression of each of these monoamine transporters have provided a wealth of new information about the function of these proteins at molecular, physiological and behavioral levels. In the present work we use the unique properties of magnetic resonance imaging (MRI) to probe the effects of altered dopaminergic dynamics on meso-scale neuronal circuitry and overall brain morphology, since changes at these levels of organization might help to account for some of the extensive pharmacological and behavioral differences observed in dopamine transporter (DAT) KO mice. Despite the smaller size of these animals, voxel-wise statistical comparison of high resolution structural MR images indicated little morphological change as a consequence of DAT KO. Likewise, proton magnetic resonance spectra recorded in the striatum indicated no significant changes in detectable metabolite concentrations between DAT KO and wild-type (WT) mice. In contrast, alterations in the circuitry from the prefrontal cortex to the mesocortical limbic system, an important brain component intimately tied to function of mesolimbic/mesocortical dopamine reward pathways, were revealed by manganese-enhanced MRI (MEMRI). Analysis of co-registered MEMRI images taken over the 26 hours after introduction of Mn^(2+) into the prefrontal cortex indicated that DAT KO mice have a truncated Mn^(2+) distribution within this circuitry with little accumulation beyond the thalamus or contralateral to the injection site. By contrast, WT littermates exhibit Mn^(2+) transport into more posterior midbrain nuclei and contralateral mesolimbic structures at 26 hr post-injection. Thus, DAT KO mice appear, at this level of anatomic resolution, to have preserved cortico-striatal-thalamic connectivity but diminished robustness of reward-modulating circuitry distal to the thalamus. This is in contradistinction to the state of this circuitry in serotonin transporter KO mice where we observed more robust connectivity in more posterior brain regions using methods identical to those employed here