Extracellular Striatal Dopamine Levels Research Articles

Stroke Literature Synopsis (Preclinical).

Stroke Literature Synopsis (Preclinical).

Diverging changes in rat striatal extracellular dopamine and DOPAC levels and in frequency-modulated 50-kHz ultrasonic vocalizations rate during repeated amphetamine treatment

One characteristic feature of addictive drugs is their ability to induce, after a single exposure, a lasting sensitization to the next doses; the underlying neuroplastic changes supposedly involve the brain dopamine system. We aimed at identifying putative relationships between alterations in extracellular dorsal striatal dopamine, HVA and DOPAC levels and in frequency-modulated 50-kHz ultrasonic vocalizations rate response during repeated intraperitoneal amphetamine treatment. Measurements were performed before and after amphetamine doses 1, 2, 7 and 8 (Amph1, Amph2, Amph7 and Amph8; treatment days 1, 7, 12 and 23, respectively). Amphetamine was confirmed to induce sensitization of the vocalization response, but an extended recording time (180 instead of 20 min) revealed that sensitization of this response requires more time to develop than hitherto believed. Baseline extracellular dopamine level increased initially, declined after a series of daily amphetamine doses and showed some tendency for recovery after drug withdrawal. Baseline extracellular DOPAC (but not HVA) showed a continuous decline during the treatment. There was no significant change in the integrated short-term (3-h) extracellular dopamine response, whereas the respective DOPAC collection lowered significantly after repeated drug treatment. Extracellular DOPAC is believed to originate mostly from newly synthesized dopamine, hence the declines in its baseline and post-amphetamine levels suggest falling dopamine synthesis. These results indicate that sensitization of the appetitive vocalization response to amphetamine continues despite reduced dorsal striatal dopamine synthesis and involves no changes in amphetamine-induced dopamine release.

A dopaminergic mechanism of antipsychotic drug efficacy, failure, and failure reversal: the role of the dopamine transporter

Antipsychotic drugs are effective interventions in schizophrenia. However, the efficacy of these agents often decreases over time, which leads to treatment failure and symptom recurrence. We report that antipsychotic efficacy in rat models declines in concert with extracellular striatal dopamine levels rather than insufficient dopamine D2 receptor occupancy. Antipsychotic efficacy was associated with a suppression of dopamine transporter activity, which was reversed during failure. Antipsychotic failure coincided with reduced dopamine neuron firing, which was not observed during antipsychotic efficacy. Synaptic field responses in dopamine target areas declined during antipsychotic efficacy and showed potentiation during failure. Antipsychotics blocked synaptic vesicle release during efficacy but enhanced this release during failure. We found that the pharmacological inhibition of the dopamine transporter rescued antipsychotic drug treatment outcomes, supporting the hypothesis that the dopamine transporter is a main target of antipsychotic drugs and predicting that dopamine transporter blockers may be an adjunct treatment to reverse antipsychotic treatment failure.

P.2.023 - Impairment of learning and memory in mice overexpressing α- and γ-synuclein in dopaminergic neurons- implication in Parkinson’s disease

Introduction Idiopathic Parkinson’s disease (PD) is characterized by progressive brain pathology affecting several neurotransmitter systems, leading to a diverse profile of autonomic, motor, cognitive and psychiatric dysfunctions. Multiple convergent lines of evidence implicate the aggregation and accumulation of α-synuclein in the PD pathogenesis [1]. Moreover, post-translational changes of γ-synuclein increase the α-synuclein aggregation [2]. α- and γ-synuclein are members of the synuclein family of small cytoplasmic neuron-specific proteins, and have been suggested to modulate the function of monoamine transporters (DAT, SERT, NET) [3]. Previously, we reported that α-synuclein knockdown mice showed increased extracellular dopamine and serotonin levels –compared to control mice– in caudate-putamen (CPu) and medial prefrontal cortex (mPFC) after DAT or SERT blockade with nomifensine or citalopram, respectively (Alarcon-Aris et al., in review). Aims Given the role of monoamine neurotransmission in cognitive processes, in the present study we assessed the effects of over-expressing α- and γ-synuclein in mouse dopaminergic neurons on spatial and working memory. Methods We generated a mouse model overexpressing human wild-type α-synucleinor endogenous γ-synuclein in dopaminergic neurons from the substantia nigra compacta (SNc) and ventral tegmental area (VTA) using adeno-associated virus type-5 (AAV5-human-α-synuclein) or type-10 (AAV10-mouse-γ-synuclein) unilaterally injected in SNc/VTA. Behavioral tests were performed in mice at 4, 8 and 16 weeks post-infection using cognitive (novel object recognition, Morris water maze, passive avoidance tests) and motor paradigms (cylinder test, rotarod). Extracellular striatal dopamine levels were measured by in vivo microdialysis after implanting concentric dialysis probes, and analyzed by high precision liquid chromatography with electrochemical detection (+0.75 V). In situ hybridization was performed on fixed mice tissues, with 33P-dATP labeled probes in order to detect α- and γ-synuclein transcripts through autoradiography. Results AAV5-α-synuclein mice showed increased human α-synuclein mRNA levels in the ipsilateral SNc over time reaching a 278% of sham mice at 8 weeks post-infection. Similarly, AAV10-γ-synuclein mice reached to a maximal of 250% of sham mice. Overexpression of α- or γ-synuclein led to lower extracellular dopamine levels in CPu and mPFC compared to sham mice after local veratridine (sodium chanel activator agent, 50 µM) or nomifensine (DAT inhibitor, 1-10-50 µM) administration at 4 weeks post-infection. Moreover, overexpression of α- or γ-synuclein led to the impairment of working memory in mice as revealed by passive avoidance test (retention memory 35.5% and 45.9% shorter in AAV5-α-synuclein andAAV10-γ-synuclein mice, respectively) and novel object recognition test (reduction of 10% in time spent exploring novel object in both infected models) at 4-8 weeks post-infection. Likewise, these mice showed deficits in the spatial learning using the Morris water maze (F(1,28)=5.711, P=0.02 in AAV5-α-synuclein; F(1,28)=11.43, P=0.0021 in AAV10-γ-synuclein). These non-motor symptoms appeared earlier than motor deficits assessed by the cylinder test and rotarod. Conclusions Overall, these findings indicate that 1) α- and γ-synuclein are directly involved in the regulation of cognitive functions, likely through changes in monoamine functions, and 2) overexpression of α- or γ-synuclein in dopaminergic neurons mimics the temporal progression of clinical symptoms in PD, with early cognitive deficits appearing before motor symptoms.

Age-dependent dopamine transporter dysfunction and Serine129 phospho-\u03b1-synuclein overload in G2019S LRRK2 mice

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are the most common genetic cause of Parkinson’s disease. Here, we investigated whether the G2019S LRRK2 mutation causes morphological and/or functional changes at nigro-striatal dopamine neurons. Density of striatal dopaminergic terminals, nigral cell counts, tyrosine hydroxylase protein levels as well as exocytotic dopamine release measured in striatal synaptosomes, or striatal extracellular dopamine levels monitored by in vivo microdialysis were similar between ≥12-month-old G2019S knock-in mice and wild-type controls. In vivo striatal dopamine release was insensitive to the LRRK2 inhibitor Nov-LRRK2-11, and was elevated by the membrane dopamine transporter blocker GBR-12783. However, G2019S knock-in mice showed a blunted neurochemical and motor activation response to GBR-12783 compared to wild-type controls. Western blot and dopamine uptake analysis revealed an increase in dopamine transporter levels and activity in the striatum of 12-month-old G2019S KI mice. This phenotype correlated with a reduction in vesicular monoamine transporter 2 levels and an enhancement of vesicular dopamine uptake, which was consistent with greater resistance to reserpine-induced hypolocomotion. These changes were not observed in 3-month-old mice. Finally, Western blot analysis revealed no genotype difference in striatal levels of endogenous α-synuclein or α-synuclein bound to DOPAL (a toxic metabolite of dopamine). However, Serine129-phosphorylated α-synuclein levels were higher in 12-month-old G2019S knock-in mice. Immunohistochemistry confirmed this finding, also showing no genotype difference in 3-month-old mice. We conclude that the G2019S mutation causes progressive dysfunctions of dopamine transporters, along with Serine129-phosphorylated α-synuclein overload, at striatal dopaminergic terminals, which are not associated with dopamine homeostasis dysregulation or neuron loss but might contribute to intrinsic dopaminergic terminal vulnerability. We propose G2019S knock-in mice as a presymptomatic Parkinson’s disease model, useful to investigate the pathogenic interaction among genetics, aging, and internal or environmental factors leading to the disease.

Antidepressants differentially affect striatal amphetamine-stimulated dopamine and serotonin release in rats with high and low novelty-oriented behaviour

In the studies of depression pathogenesis and antidepressant action, the monoaminergic hypothesis of depression has mainly focused on the serotonergic and noradrenergic mechanisms. However, dopaminergic neurotransmission is also linked to both depressive symptomatology as well as antidepressant effects. We have previously shown that persistent inter-individual differences in the rat behavioural activity in novel environments is associated with differences in the striatal extracellular levels of dopamine and serotonin, depressive-like behaviour and the expression of several depression-related genes. The aim of the current study was to investigate the relative potency of the tricyclic antidepressant imipramine, the selective serotonin re-uptake inhibitor fluoxetine, and the selective noradrenaline re-uptake inhibitor reboxetine (all drugs administered in the dose of 10mg/kg, i.p.) to enhance amphetamine-stimulated dopamine and serotonin release in the striatum using in vivo microdialysis in awake, freely-moving rats, categorized into high explorers (HE) and low explorers (LE) based on their spontaneous novelty-oriented behaviour. The basal extracellular dopamine and serotonin concentration in the striatum did not differ between the LE- and HE-rats. None of the antidepressants alone were able to modify baseline striatal dopamine levels, but the amphetamine-stimulated dopamine release was significantly higher in the HE-rats after acute and chronic imipramine (but not fluoxetine or reboxetine). Acute imipramine and fluoxetine, but not reboxetine, increased both the basal and amphetamine-stimulated levels of serotonin in the striatum. Again, the HE-rats had higher amphetamine-stimulated serotonin release after fluoxetine administration. These findings suggest that rats with depressive-like phenotype are less sensitive to the neurochemical effects of antidepressants in the striatum. These results may have relevance in understanding the neurobiological bases for inter-individual differences in antidepressant treatment response in humans and development of novel medicines.

Fatty acid amide supplementation decreases impulsivity in young adult heavy drinkers

Compromised dopamine signaling in the striatum has been associated with the expression of impulsive behaviors in addiction, obesity and alcoholism. In rodents, intragastric infusion of the fatty acid amide oleoylethanolamide increases striatal extracellular dopamine levels via vagal afferent signaling. Here we tested whether supplementation with PhosphoLean™, a dietary supplement that contains the precursor of the fatty acid amide oleoylethanolamide (N-oleyl-phosphatidylethanolamine), would reduce impulsive responding and alcohol use in heavy drinking young adults. Twenty-two individuals were assigned to a three-week supplementation regimen with PhosphoLean™ or placebo. Impulsivity was assessed with self-report questionnaires and behavioral tasks pre- and post-supplementation. Although self-report measures of impulsivity did not change, supplementation with PhosphoLean™, but not placebo, significantly reduced false alarm rate on a Go/No-Go task. In addition, an association was found between improved sensitivity on the Go/No-Go task and reduced alcohol intake. These findings provide preliminary evidence that promoting fatty acid derived gut-brain dopamine communication may have therapeutic potential for reducing impulsivity in heavy drinkers.

Orbitofrontal dopamine depletion upregulates caudate dopamine and alters behavior via changes in reinforcement sensitivity.

Schizophrenia is associated with upregulation of dopamine (DA) release in the caudate nucleus. The caudate has dense connections with the orbitofrontal cortex (OFC) via the frontostriatal loops, and both areas exhibit pathophysiological change in schizophrenia. Despite evidence that abnormalities in dopaminergic neurotransmission and prefrontal cortex function co-occur in schizophrenia, the influence of OFC DA on caudate DA and reinforcement processing is poorly understood. To test the hypothesis that OFC dopaminergic dysfunction disrupts caudate dopamine function, we selectively depleted dopamine from the OFC of marmoset monkeys and measured striatal extracellular dopamine levels (using microdialysis) and dopamine D2/D3 receptor binding (using positron emission tomography), while modeling reinforcement-related behavior in a discrimination learning paradigm. OFC dopamine depletion caused an increase in tonic dopamine levels in the caudate nucleus and a corresponding reduction in D2/D3 receptor binding. Computational modeling of behavior showed that the lesion increased response exploration, reducing the tendency to persist with a recently chosen response side. This effect is akin to increased response switching previously seen in schizophrenia and was correlated with striatal but not OFC D2/D3 receptor binding. These results demonstrate that OFC dopamine depletion is sufficient to induce striatal hyperdopaminergia and changes in reinforcement learning relevant to schizophrenia.

SK channel blockade reverses cognitive and motor deficits induced by nigrostriatal dopamine lesions in rats

Parkinson's disease has traditionally been viewed as a motor disorder caused by the loss of dopamine (DA) neurons. However, emotional and cognitive syndromes can precede the onset of the motor deficits and provide an opportunity for therapeutic intervention. Potassium channels have recently emerged as potential new targets in the treatment of Parkinson's disease. The selective blockade of small conductance calcium-activated K+ channels (SK channels) by apamin is known to increase burst firing in midbrain DA neurons and therefore DA release. We thus investigated the effects of systemic administration of apamin on the motor, cognitive deficits and anxiety present after bilateral nigrostriatal 6-hydroxydopamine (6-OHDA) lesions in rats. Apamin administration (0.1 or 0.3mg/kg i.p.) counteracted the depression, anxiety-like behaviors evaluated on sucrose consumption and in the elevated plus maze, social recognition and spatial memory deficits produced by partial 6-OHDA lesions. Apamin also reduced asymmetric motor deficits on circling behavior and postural adjustments in the unilateral extensive 6-OHDA model. The partial 6-OHDA lesions (56% striatal DA depletion) produced 20% decrease of iodinated apamin binding sites in the substantia nigra pars compacta in correlation with the loss of tyrosine hydroxylase positive cells, without modifying apamin binding in brain regions receiving DAergic innervation. Striatal extracellular levels of DA, not detectable after 6-OHDA lesions, were enhanced by apamin treatment as measured by in vivo microdialysis. These results indicate that blocking SK channels may reinstate minimal DA activity in the striatum to alleviate the non-motor symptoms induced by partial striatal DA lesions.

Potentiated Striatal Dopamine Release Leads to Hyperdopaminergia in Female Brain-Derived Neurotrophic Factor Heterozygous Mice

The goal of this study was to determine whether a reduction in brain-derived neurotrophic factor (BDNF) levels in female mice leads to dopaminergic system dysregulation. Through a series of in vivo brain microdialysis and slice voltammetry experiments, we discerned that female BDNF heterozygous (BDNF(+/-)) mice are hyperdopaminergic, similar to their male BDNF(+/-) counterparts. Zero-net flux microdialysis results showed that female BDNF(+/-) mice had increased striatal extracellular dopamine levels, while stimulated regional release by high potassium concentrations potentiated dopamine release through vesicular-mediated depolarization. Using the complementary technique of fast scan cyclic voltammetry, electrical stimulation evoked greater dopamine release in the female BDNF(+/-) mice, whereas dopamine uptake remained unchanged relative to that of female wildtype mice. Following psychostimulant methamphetamine administration, female BDNF(+/-) mice showed potentiated dopamine release compared to their wildtype counterparts. Taken together, these dopamine release impairments in female mice appear to result in a hyperdopaminergic phenotype without concomitant alterations in dopamine uptake.

4-Methylthioamphetamine Increases Dopamine in the Rat Striatum and has Rewarding EffectsIn Vivo

4-Methylthioamphetamine (MTA) is a phenylisopropylamine derivative whose use has been associated with severe intoxications. MTA is usually regarded as a selective serotonin-releasing agent. Nevertheless, previous data have suggested that its mechanism of action probably involves a catecholaminergic component. As little is known about dopaminergic effects of this drug, in this work the actions of MTA upon the dopamine (DA) transporter (DAT) were studied in vitro, in vivo and in silico. Also, the possible abuse liability of MTA was behaviourally assessed. MTA exhibited an in vitro affinity for the rat DAT in the low micromolar range (6.01μM) and induced a significant, dose-dependent increase in striatal DA. MTA significantly increased c-Fos-positive cells in striatum and nucleus accumbens, induced conditioned place preference and increased locomotor activity. Docking experiments were performed in a homology model of the DAT. In conclusion, our results show that MTA is able to increase extracellular striatal DA levels and that its administration has rewarding properties. These effects were observed at concentrations or doses that can be relevant to its use in human beings.

Regulation of fat intake in the absence of flavour signalling

Animals, including humans, can achieve precise regulation of caloric intake by adjusting consumption in response to covert changes in energy density. It remains unknown, however, whether the presence of flavour cues are required for the ability to maintain constant caloric intake. Also unknown are the brain circuits that may function as the central calorie monitors that control adaptive adjustments in energy intake. Here we show that mice trained to lick a dry spout in order to receive intra-gastric infusions of a fat emulsion maintained constant hourly caloric intake by adjusting the number of dry licks in response to changes in caloric density. Animals also increased dry licking according to hunger levels, and developed conditioned preferences for dry sippers associated with high calorie infusions. Importantly, striatal dopamine levels were closely associated with the amount of calories ingested, rather than with the number of dry licks produced. Dopamine levels in dorsal and ventral striatum also reflected caloric density in mice passively receiving intra-gastric infusions of fat emulsions. Consistent with the above, systemic administration of the dopamine receptor blocker haloperidol markedly increased the production of dry licks needed to obtain high-calorie infusions, as if the caloric density of the infusions had been diluted. Conversely, haloperidol markedly decreased the production of dry licks needed to obtain low-calorie infusions. Taken together, our results support the proposition that brain dopamine circuits function as one central sensor of calorie ingestion, since (1) extracellular striatal dopamine levels fluctuate in proportion to the caloric density of nutrients infused in the gut; and (2) inhibiting dopamine receptor signalling disrupts the animals' ability to maintain constant caloric intake across experimental sessions.

Contribution of pre-synaptic mechanisms to l-DOPA-induced dyskinesia

Positron emission tomography (PET) imaging studies have shown that peak-dose dyskinesia is associated to abnormally high levels of synaptic dopamine (DA) in the caudate-putamen of dyskinetic L-DOPA-treated patients. High striatal extracellular DA levels have also been found in dyskinetic 6-OHDA-lesioned rats as compared to non-dyskinetic ones, suggesting that extracellular DA levels may play a key role in the induction of dyskinesia. In this article we review the evidences pointing to the serotonin system as the primary cause for the abnormally high levels of L-DOPA-derived extracellular DA in Parkinson's disease, and we discuss the feasibility of a therapeutic approach targeting this system.

Mechanisms underlying altered striatal synaptic plasticity in old A53T-α synuclein overexpressing mice

The interactions between certain α-synuclein (SNCA) conformations and dopamine (DA) metabolism cause selective DA neuron degeneration in Parkinson's disease (PD). Preclinical research on PD took advantage of increasing studies involving different animal models which express different forms of mutated SNCA. Transgenic animals expressing mutant α-synucleins such as mice transgenic for A53T-SNCA (TG) are considered valuable models to assess specific aspects of the pathogenesis of synucleinopathies and PD. In this study we performed electrophysiological recordings in corticostriatal slice preparations from young TG overexpressing mice, in which extracellular striatal DA levels appeared to be normal, and in old TG mice, characterized by abnormalities in striatal DA signaling and impaired long-term depression (LTD). We report no difference in TG mice from the two groups of age of either the basal membrane properties and synaptic striatal excitability in respect to age-matched wild-type mice. Furthermore, in old TG mice, showing plastic abnormalities and motor symptoms, we investigated the mechanisms at the basis of the altered LTD. In old TG mice LTD could not be restored by treatments with acute application of DA or by subchronic treatment with L-3,4-dihydroxyphenylalanine (L-DOPA). Conversely, the application of the phosphodiesterase inhibitor zaprinast fully restored LTD to normal conditions via the stimulation of a cyclic guanosine monophosphate (GMP)-protein kinase G-dependent intracellular signaling pathway. These results suggest that, in addition to the dopaminergic alterations reported in this genetic model of PD, other signal transduction pathways linked to striatal synaptic plasticity are altered in an age-dependent manner.

Elevated tonic extracellular dopamine concentration and altered dopamine modulation of synaptic activity precede dopamine loss in the striatum of mice overexpressing human α‐synuclein

Overexpression or mutation of α-synuclein (α-Syn), a protein associated with presynaptic vesicles, causes familial forms of Parkinson's disease in humans and is also associated with sporadic forms of the disease. We used in vivo microdialysis, tissue content analysis, behavioral assessment, and whole-cell patch clamp recordings from striatal medium-sized spiny neurons (MSSNs) in slices to examine dopamine transmission and dopaminergic modulation of corticostriatal synaptic function in mice overexpressing human wild-type α-Syn under the Thy1 promoter (α-Syn mice). Tonic striatal extracellular dopamine and 3-methoxytyramine levels were elevated in α-Syn mice at 6 months of age, prior to any reduction in total striatal tissue content, and were accompanied by an increase in open-field activity. Dopamine clearance and amphetamine-induced dopamine efflux were unchanged. The frequency of MSSN spontaneous excitatory postsynaptic currents (sEPSCs) was lower in α-Syn mice. Amphetamine reduced sEPSC frequency in wild types (WTs) but produced no effect in α-Syn mice. Furthermore, whereas quinpirole reduced and sulpiride increased sEPSC frequency in WT mice, they produced the opposite effects in α-Syn mice. These observations indicate that overexpression of α-Syn alters dopamine efflux and D2 receptor modulation of corticostriatal glutamate release at a young age. At 14 months of age, the α-Syn mice presented with significantly lower striatal tissue dopamine and tyrosine hydroxylase content relative to WT littermates, accompanied by an L-DOPA-reversible sensory motor deficit. Together, these data further validate this transgenic mouse line as a slowly progressing model of Parkinson's disease and provide evidence for early dopamine synaptic dysfunction prior to loss of striatal dopamine.

Ephedrine decreases vesicular monoamine transporter‐2 function

Ephedrine decreases vesicular monoamine transporter‐2 function

Role of Aberrant Striatal Dopamine D1Receptor/cAMP/Protein Kinase A/DARPP32 Signaling in the Paradoxical Calming Effect of Amphetamine

Attention deficit/hyperactivity disorder (ADHD) is characterized by inattention, impulsivity, and motor hyperactivity. Several lines of research support a crucial role for the dopamine transporter (DAT) gene in this psychiatric disease. Consistently, the most commonly prescribed medications in ADHD treatment are stimulant drugs, known to preferentially act on DAT. Recently, a knock-in mouse [DAT-cocaine insensitive (DAT-CI)] has been generated carrying a cocaine-insensitive DAT that is functional but with reduced dopamine uptake function. DAT-CI mutants display enhanced striatal extracellular dopamine levels and basal motor hyperactivity. Herein, we showed that DAT-CI animals present higher striatal dopamine turnover, altered basal phosphorylation state of dopamine and cAMP-regulated phosphoprotein 32 kDa (DARPP32) at Thr75 residue, but preserved D(2) receptor (D(2)R) function. However, although we demonstrated that striatal D(1) receptor (D(1)R) is physiologically responsive under basal conditions, its stimulus-induced activation strikingly resulted in paradoxical electrophysiological, behavioral, and biochemical responses. Indeed, in DAT-CI animals, (1) striatal LTP was completely disrupted, (2) R-(+)-6-chloro-7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide (SKF 81297) treatment induced paradoxical motor calming effects, and (3) SKF 81297 administration failed to increase cAMP/protein kinase A (PKA)/DARPP32 signaling. Such biochemical alteration selectively affected dopamine D(1)Rs since haloperidol, by blocking the tonic inhibition of D(2)R, unmasked a normal activation of striatal adenosine A(2A) receptor-mediated cAMP/PKA/DARPP32 cascade in mutants. Most importantly, our studies highlighted that amphetamine, nomifensine, and bupropion, through increased striatal dopaminergic transmission, are able to revert motor hyperactivity of DAT-CI animals. Overall, our results suggest that the paradoxical motor calming effect induced by these drugs in DAT-CI mutants depends on selective aberrant phasic activation of D(1)R/cAMP/PKA/DARPP32 signaling in response to increased striatal extracellular dopamine levels.

Autoradiographic and pharmacological studies on the role of dopamine D3 receptors in genetically dystonic ( dtsz) hamsters

Previous examinations demonstrated periodic increases in striatal extracellular dopamine levels during dystonic attacks and changes in dopamine D1 and D2 receptor binding in the dt sz mutant hamster, an animal model of paroxysmal non-kinesiogenic dyskinesia in which dystonic episodes can be induced by stress. Since dopamine D3 receptors are involved in the regulation of striatal dopamine release, D3 receptor function was investigated by autoradiographic and pharmacological examinations in mutant hamsters in the present study. [ 125I]7-[[(E)-3-iodoprop-2-enyl]-propylamino]-5,6,7,8-tetrahydronaphthalen-2-ol ([ 125I]7-OH-PIPAT) binding was not significantly altered in the striatum, n. accumbens, ventral pallidum or cerebellum in dt sz hamsters in comparison to non-dystonic control hamsters. In line with the unaltered D3 receptor binding, the preferential dopamine D3 versus D2 receptor antagonist U-99194 (5,6-dimethoxy- N, N-dipropyl-2,3-dihydro-1H-inden-2-amine hydrochloride) did not exert significant effects on the severity of dystonia in dt sz hamsters at doses of 10 to 40 mg/kg which induced hyperlocomotion. These results suggest that periodic elevations of dopamine levels in these animals are not related to D3 receptor dysfunctions.

Dopamine transporter-related effects of modafinil in rhesus monkeys

Modafinil is currently used as a treatment for daytime sleepiness. The objectives of this study were to explore the dopamine transporter (DAT)-related effects of modafinil on behavior and in vivo neurochemistry in rhesus monkeys (Macaca mulatta). The effects of modafinil (3.0-10 mg/kg, i.v.) were evaluated on locomotor activity, reinstatement of cocaine-maintained behavior, extracellular dopamine levels in the caudate nucleus, and DAT occupancy in the dorsal striatum. Eight subjects were fitted with a collar-mounted activity monitor to evaluate sleep-activity cycles, with 4 days of baseline recording preceding an injection of saline or modafinil (3.0-10 mg/kg). The effects of modafinil (3.0-10 mg/kg) and cocaine (0.3 mg/kg) on reinstatement of behavior that was previously maintained under a second-order schedule of i.v. cocaine delivery were tested in a separate group of subjects (n = 6). Finally, the effects of modafinil (3.0-10 mg/kg) on extracellular dopamine levels and DAT occupancy in vivo were characterized using microdialysis and positron emission tomography, respectively, in a within-subjects design (n = 4). Modafinil significantly increased nighttime locomotor activity and reinstated cocaine-maintained behavior but did not affect daytime locomotor activity. Modafinil significantly increased striatal extracellular dopamine levels at a dose that resulted in DAT occupancy of 64.4% (putamen) and 60.2% (caudate). The behavioral and in vivo dopaminergic effects of modafinil are consistent with the profile of a low potency DAT inhibitor and may indicate potential for abuse at high doses.

Hyperactivity of the dopaminergic system in NTS1 and NTS2 null mice

Neurotensin (NT) is a tridecapeptide that acts as a neuromodulator in the central nervous system mainly through two NT receptors, NTS1 and NTS2. The functional-anatomical interactions between NT, the mesotelencephalic dopamine system, and structures targeted by dopaminergic projections have been studied. The present study was conducted to determine the effects of NT receptor subtypes on dopaminergic function with the use of mice lacking either NTS1 (NTS1(-/-)) or NTS2 (NTS2(-/-)). Basal and amphetamine-stimulated locomotor activity was determined. In vivo microdialysis in freely moving mice, coupled with HPLC-ECD, was used to detect basal and d-amphetamine-stimulated striatal extracellular dopamine levels. In vitro radioligand binding and synaptosomal uptake assays for the dopamine transporters were conducted to test for the expression and function of the striatal pre-synaptic dopamine transporter. NTS1(-/-) and NTS2(-/-) mice had higher baseline locomotor activity and higher basal extracellular dopamine levels in striatum. NTS1(-/-) mice showed higher locomotor activity and exaggerated dopamine release in response to d-amphetamine. Both NTS1(-/-) and NTS2(-/-) mice exhibited lower dopamine D(1) receptor mRNA expression in the striatum relative to wild type mice. Dopamine transporter binding and dopamine reuptake in striatum were not altered. Therefore, lack of either NTS1 or NTS2 alters the dopaminergic system. The possibility that the dysregulation of dopamine transmission might stem from a deficiency in glutamate neurotransmission is discussed. The data strengthen the hypothesis that NT receptors are involved in the pathogenesis of schizophrenia and provide a potential model for the biochemical changes of the disease.