Fluoxetine reduces L-DOPA-derived extracellular DA in the 6-OHDA-lesioned rat striatum

Role of 5-HT1A Receptor in Vilazodone-Mediated Suppression of L-DOPA-Induced Dyskinesia and Increased Responsiveness to Cortical Input in Striatal Medium Spiny Neurons in an Animal Model of Parkinson's Disease

L-DOPA therapy in Parkinson’s disease (PD) is limited due to emerging L-DOPA-induced dyskinesia. Research has identified abnormal dopamine release from serotonergic (5-HT) terminals contributing to this dyskinesia. Selective serotonin reuptake inhibitors (SSRIs) or 5-HT receptor (5-HTr) agonists can regulate 5-HT activity and attenuate dyskinesia, but they often also produce a loss of the antiparkinsonian efficacy of L-DOPA. We investigated vilazodone, a novel multimodal 5-HT agent with SSRI and 5-HTr_1A partial agonist properties, for its potential to reduce dyskinesia without interfering with the prokinetic effects of L-DOPA, and underlying mechanisms. We assessed vilazodone effects on L-DOPA-induced dyskinesia (abnormal involuntary movements, AIMs) and aberrant responsiveness to corticostriatal drive in striatal medium spiny neurons (MSNs) measured with in vivo single-unit extracellular recordings, in the 6-OHDA rat model of PD. Vilazodone (10 mg/kg) suppressed all subtypes (axial, limb, orolingual) of AIMs induced by L-DOPA (5 mg/kg) and the increase in MSN responsiveness to cortical stimulation (shorter spike onset latency). Both the antidyskinetic effects and reversal in MSN excitability by vilazodone were inhibited by the 5-HTr_1A antagonist WAY-100635, demonstrating a critical role for 5-HTr_1A in these vilazodone actions. Our results indicate that vilazodone may serve as an adjunct therapeutic for reducing dyskinesia in patients with PD.

The Multimodal Serotonergic Agent Vilazodone Inhibits L-DOPA-Induced Gene Regulation in Striatal Projection Neurons and Associated Dyskinesia in an Animal Model of Parkinson's Disease

Levodopa (L-DOPA) treatment in Parkinson's disease is limited by the emergence of L-DOPA-induced dyskinesia. Such dyskinesia is associated with aberrant gene regulation in neurons of the striatum, which is caused by abnormal dopamine release from serotonin terminals. Previous work showed that modulating the striatal serotonin innervation with selective serotonin reuptake inhibitors (SSRIs) or 5-HT1A receptor agonists could attenuate L-DOPA-induced dyskinesia. We investigated the effects of a novel serotonergic agent, vilazodone, which combines SSRI and 5-HT1A partial agonist properties, on L-DOPA-induced behavior and gene regulation in the striatum in an animal model of Parkinson's disease. After unilateral dopamine depletion by 6-hydroxydopamine (6-OHDA), rats received repeated L-DOPA treatment (5 mg/kg) alone or in combination with vilazodone (10 mg/kg) for 3 weeks. Gene regulation was then mapped throughout the striatum using in situ hybridization histochemistry. Vilazodone suppressed the development of L-DOPA-induced dyskinesia and turning behavior but did not interfere with the prokinetic effects of L-DOPA (forelimb stepping). L-DOPA treatment drastically increased the expression of dynorphin (direct pathway), 5-HT1B, and zif268 mRNA in the striatum ipsilateral to the lesion. These effects were inhibited by vilazodone. In contrast, vilazodone had no effect on enkephalin expression (indirect pathway) or on gene expression in the intact striatum. Thus, vilazodone inhibited L-DOPA-induced gene regulation selectively in the direct pathway of the dopamine-depleted striatum, molecular changes that are considered critical for L-DOPA-induced dyskinesia. These findings position vilazodone, an approved antidepressant, as a potential adjunct medication for the treatment of L-DOPA-induced motor side effects.

Serotonergic targets for the treatment of L-DOPA-induced dyskinesia

Dopamine (DA) replacement therapy with L-3,4-dihydroxyphenylalanine (L-DOPA) continues to be the gold-standard treatment for Parkinson's disease (PD). Despite clear symptomatic benefit, long-term L-DOPA use often results in the development of L-DOPA-induced dyskinesia (LID), significantly reducing quality of life and increasing costs for PD patients and their caregivers. Accumulated research has demonstrated that several pre- and post-synaptic mechanisms contribute to LID development and expression. In particular, raphe-striatal hyperinnervation and unregulated DA release from 5-HT terminals is postulated to play a central role in LID manifestation. As such, manipulation of the 5-HT system has garnered considerable attention. Both pre-clinical and clinical research has supported the potential of modulating the 5-HT system for LID prevention and treatment. This review discusses the rationale for continued investigation of several potential anti-dyskinetic strategies including 5-HT stimulation of 5-HT1A and 5-HT1B receptors and blockade of 5-HT2A receptors and SERT. We present the latest findings from experimental and clinical investigations evaluating these 5-HT targets with the goal of identifying those with translational promise and the challenges associated with each.

Impairment of Serotonergic Transmission by the Antiparkinsonian Drug L-DOPA: Mechanisms and Clinical Implications

The link between the anti-Parkinsonian drug L-3,4-dihydroxyphenylalanine (L-DOPA) and the serotonergic (5-HT) system has been long established and has received increased attention during the last decade. Most studies have focused on the fact that L-DOPA can be transformed into dopamine (DA) and released from 5-HT terminals, which is especially important for the management of L-DOPA-induced dyskinesia. In patients, treatment using L-DOPA also impacts 5-HT neurotransmission; however, few studies have investigated the mechanisms of this effect. The purpose of this review is to summarize the electrophysiological and neurochemical data concerning the effects of L-DOPA on 5-HT cell function. This review will argue that L-DOPA disrupts the link between the electrical activity of 5-HT neurons and 5-HT release as well as that between 5-HT release and extracellular 5-HT levels. These effects are caused by the actions of L-DOPA and DA in 5-HT neurons, which affect 5-HT neurotransmission from the biosynthesis of 5-HT to the impairment of the 5-HT transporter. The interaction between L-DOPA and 5-HT transmission is especially relevant in those Parkinson’s disease (PD) patients that suffer dyskinesia, comorbid anxiety or depression, since the efficacy of antidepressants or 5-HT compounds may be affected.

What Mechanisms Are Responsible for the Reuptake of Levodopa-Derived Dopamine in Parkinsonian Striatum?

Levodopa is the most effective medication for motor symptoms in Parkinson's disease. However, various motor and non-motor complications are associated with levodopa treatment, resulting from altered levodopa-dopamine metabolism with disease progression and long-term use of the drug. The present review emphasizes the role of monoamine transporters other than the dopamine transporter in uptake of extracellular dopamine in the dopamine-denervated striatum. When dopaminergic neurons are lost and dopamine transporters decreased, serotonin and norepinephrine transporters compensate by increasing uptake of excessive extracellular dopamine in the striatum. Organic cation transporter-3 and plasma membrane monoamine transporter, low affinity, and high capacity transporters, also potentially uptake dopamine when high-affinity transporters do not work normally. Selective serotonin reuptake inhibitors and serotonin norepinephrine reuptake inhibitors are often administered to patients with Parkinson's disease presenting with depression, pain or other non-motor symptoms. Thus, it is important to address the potential of these drugs to modify dopamine metabolism and uptake through blockade of the compensatory function of these transporters, which could lead to changes in motor symptoms of Parkinson's disease.

L-DOPA elicits non-vesicular releases of serotonin and dopamine in hemiparkinsonian rats in vivo

The control of the secretory activity of serotonergic neurons has been pointed out to reduce motor and non-motor side effects of the antiparkinsonian drug L-DOPA. This strategy deserves further investigation because it is presently unclear whether L-DOPA promotes a non-vesicular release of dopamine and serotonin from serotonergic neurons. To get a full neurochemical picture compatible with the existence of such a mechanism, we combined multisite intracerebral microdialysis, post mortem tissue measurement and single unit extracellular recordings in the dorsal raphe nucleus from hemiparkinsonian rats. L-DOPA (3-100mg/kg, ip.) non-homogeneously decreased extracellular serotonin levels in the striatum, substantia nigra pars reticulata, hippocampus and prefrontal cortex and homogenously serotonin tissue content in the striatum, cortex and cerebellum. L-DOPA (12mg/kg) did not modify the firing rate or pattern of serotonergic-like neurons recorded in the dorsal raphe nucleus. When focusing on serotonin release in the prefrontal cortex and the hippocampus, we found that L-DOPA (12 or 100mg/kg) enhanced serotonin extracellular levels in both regions upon Ca(2+) removal. Concomitantly, L-DOPA-stimulated dopamine release partly persisted in the absence of Ca(2+) in a region-dependent manner. Local application of the serotonin reuptake inhibitor citalopram (1µM) blunted the responses to L-DOPA (3-12mg/kg), measured as extracellular dopamine levels, most prominently in the hippocampus. These data stress that L-DOPA, already at low to moderate doses, promotes non-vesicular releases of serotonin and dopamine in a region-dependent manner.

Expanding the repertoire of L-DOPA's actions: A comprehensive review of its functional neurochemistry

Though a multi-facetted disorder, Parkinson's disease is prototypically characterized by neurodegeneration of nigrostriatal dopaminergic neurons of the substantia nigra pars compacta, leading to a severe disruption of motor function. Accordingly, L-DOPA, the metabolic precursor of dopamine (DA), is well-established as a treatment for the motor deficits of Parkinson's disease despite long-term complications such as dyskinesia and psychiatric side-effects. Paradoxically, however, despite the traditional assumption that L-DOPA is transformed in residual striatal dopaminergic neurons into DA, the mechanism of action of L-DOPA is neither simple nor entirely clear. Herein, focussing on its influence upon extracellular DA and other neuromodulators in intact animals and experimental models of Parkinson's disease, we highlight effects other than striatal generation of DA in the functional profile of L-DOPA. While not excluding a minor role for glial cells, L-DOPA is principally transformed into DA in neurons yet, interestingly, with a more important role for serotonergic than dopaminergic projections. Moreover, in addition to the striatum, L-DOPA evokes marked increases in extracellular DA in frontal cortex, nucleus accumbens, the subthalamic nucleus and additional extra-striatal regions. In considering its functional profile, it is also important to bear in mind the marked (probably indirect) influence of L-DOPA upon cholinergic, GABAergic and glutamatergic neurons in the basal ganglia and/or cortex, while anomalous serotonergic transmission is incriminated in the emergence of L-DOPA elicited dyskinesia and psychosis. Finally, L-DOPA may exert intrinsic receptor-mediated actions independently of DA neurotransmission and can be processed into bioactive metabolites. In conclusion, L-DOPA exerts a surprisingly complex pattern of neurochemical effects of much greater scope that mere striatal transformation into DA in spared dopaminergic neurons. Their further experimental and clinical clarification should help improve both L-DOPA-based and novel strategies for controlling the motor and other symptoms of Parkinson's disease.

The acute and long-term L-DOPA effects are independent from changes in the activity of dorsal raphe serotonergic neurons in 6-OHDA lesioned rats

BACKGROUND AND PURPOSE

L-DOPA is still the most efficacious pharmacological treatment for Parkinson's disease. However, in the majority of patients receiving long-term therapy with L-DOPA, its efficacy is compromised by motor complications, notably L-DOPA-induced dyskinesia. Evidence suggests that the serotonergic system is involved in the therapeutic and the side effects of L-DOPA. Here, we investigate if long-term L-DOPA treatment alters the activity of the dorsal raphe nucleus (DRN) and its responses to serotonergic drugs.

EXPERIMENTAL APPROACH

We measured the responses of serotonergic neurons to acute and chronic L-DOPA treatment using in vivo electrophysiological single unit-extracellular recordings in the 6-OHDA-lesion rat model of Parkinson's disease.

KEY RESULTS

The results showed that neither acute nor chronic L-DOPA administration (6 mg·kg(-1)  s.c.) altered the properties of serotonergic-like neurons. Furthermore, no correlation was found between the activity of these neurons and the magnitude of L-DOPA-induced dyskinesia. In dyskinetic rats, the inhibitory response induced by the 5-HT1A receptor agonist 8-OH-DPAT (0.0625-16 μg·kg(-1) , i.v.) was preserved. Nonetheless, L-DOPA impaired the ability of the serotonin reuptake inhibitor fluoxetine (0.125-8 mg·kg(-1) , i.v) to inhibit DRN neuron firing rate in dyskinetic animals.

CONCLUSIONS AND IMPLICATIONS

Although serotonergic neurons are involved in the dopaminergic effects of L-DOPA, we provide evidence that the effect of L-DOPA is not related to changes of the activity of DRN neurons. Rather, L-DOPA might reduce the efficacy of drugs that normally enhance the extracellular levels of serotonin.

LINKED ARTICLES

This article is part of a themed section on Updating Neuropathology and Neuropharmacology of Monoaminergic Systems. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v173.13/issuetoc.

Serotonergic modulation of the activity of mesencephalic dopaminergic systems: Therapeutic implications

Since their discovery in the mammalian brain, it has been apparent that serotonin (5-HT) and dopamine (DA) interactions play a key role in normal and abnormal behavior. Therefore, disclosure of this interaction could reveal important insights into the pathogenesis of various neuropsychiatric diseases including schizophrenia, depression and drug addiction or neurological conditions such as Parkinson's disease and Tourette's syndrome. Unfortunately, this interaction remains difficult to study for many reasons, including the rich and widespread innervations of 5-HT and DA in the brain, the plethora of 5-HT receptors and the release of co-transmitters by 5-HT and DA neurons. The purpose of this review is to present electrophysiological and biochemical data showing that endogenous 5-HT and pharmacological 5-HT ligands modify the mesencephalic DA systems' activity. 5-HT receptors may control DA neuron activity in a state-dependent and region-dependent manner. 5-HT controls the activity of DA neurons in a phasic and excitatory manner, except for the control exerted by 5-HT_2C receptors which appears to also be tonically and/or constitutively inhibitory. The functional interaction between the two monoamines will also be discussed in view of the mechanism of action of antidepressants, antipsychotics, anti-Parkinsonians and drugs of abuse.

Monoamine reuptake inhibitors in Parkinson's disease

The motor manifestations of Parkinson's disease (PD) are secondary to a dopamine deficiency in the striatum. However, the degenerative process in PD is not limited to the dopaminergic system and also affects serotonergic and noradrenergic neurons. Because they can increase monoamine levels throughout the brain, monoamine reuptake inhibitors (MAUIs) represent potential therapeutic agents in PD. However, they are seldom used in clinical practice other than as antidepressants and wake-promoting agents. This review article summarises all of the available literature on use of 50 MAUIs in PD. The compounds are divided according to their relative potency for each of the monoamine transporters. Despite wide discrepancy in the methodology of the studies reviewed, the following conclusions can be drawn: (1) selective serotonin transporter (SERT), selective noradrenaline transporter (NET), and dual SERT/NET inhibitors are effective against PD depression; (2) selective dopamine transporter (DAT) and dual DAT/NET inhibitors exert an anti-Parkinsonian effect when administered as monotherapy but do not enhance the anti-Parkinsonian actions of L-3,4-dihydroxyphenylalanine (L-DOPA); (3) dual DAT/SERT inhibitors might enhance the anti-Parkinsonian actions of L-DOPA without worsening dyskinesia; (4) triple DAT/NET/SERT inhibitors might exert an anti-Parkinsonian action as monotherapy and might enhance the anti-Parkinsonian effects of L-DOPA, though at the expense of worsening dyskinesia.

Noradrenergic terminals regulate L-DOPA-derived dopamine extracellular levels in a region-dependent manner in Parkinsonian rats

AIMS

Serotonin (5-HT) neurons mediate the ectopic release of dopamine (DA) induced by L-DOPA in the Parkinsonian brain. We hypothesized that the participation of noradrenalin transporters (NET) in the clearance of DA may account for the lower effect of L-DOPA in extrastriatal regions compared with the striatum.

METHODS

Using a multisite intracerebral microdialysis approach, we tested the influence of the pharmacological blockade of NET and/or the destruction of noradrenalin (NE) fibers on DA and 5-HT release in the striatum, hippocampus (HIPP), substantia nigra pars reticulata (SNr) and prefrontal cortex (PFC) of 6-hydroxydopamine-lesioned rats.

RESULTS

L-DOPA (12 mg/kg, i.p.) increased DA extracellular levels to a lesser extent in the SNr, PFC and HIPP compared with the striatum. The NET blockers desipramine (10 mg/kg, i.p.) and reboxetine (3 mg/kg, i.p.) potentiated L-DOPA effect in the PFC, SNr and HIPP but not in the striatum. The NE neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (50 mg/kg, i.p. 1 week before dialysis experiment) potentiated L-DOPA effect in the SNr and HIPP. 5-HT extracellular levels were enhanced only when L-DOPA was combined to NET blockers.

CONCLUSION

Noradrenalin neurons are indirectly involved in the mechanism of action of L-DOPA in part through the heterologous reuptake of DA in extrastriatal regions.

Effects of prolonged selective serotonin reuptake inhibition on the development and expression of L-DOPA-induced dyskinesia in hemi-parkinsonian rats

Dopamine (DA) replacement therapy with l-DOPA is the standard treatment for Parkinson's disease (PD). Unfortunately chronic treatment often leads to the development of abnormal involuntary movements (AIMs) referred to as L-DOPA-induced dyskinesia (LID). Accumulating evidence has shown that compensatory plasticity in serotonin (5-HT) neurons contributes to LID and recent work has indicated that acute 5-HT transporter (SERT) blockade provides anti-dyskinetic protection. However neither the persistence nor the mechanism(s) of these effects have been investigated. Therefore the current endeavor sought to mimic a prolonged regimen of SERT inhibition in L-DOPA-primed and -naïve hemi-parkinsonian rats. Rats received 3 weeks of daily co-treatment of the selective 5-HT reuptake inhibitors (SSRIs) citalopram (0, 3, or 5 mg/kg) or paroxetine (0, 0.5, or 1.25 mg/kg) with L-DOPA (6 mg/kg) during which AIMs and motor performance were monitored. In order to investigate potential mechanisms of action, tissue levels of striatal monoamines were monitored and the 5-HT(1A) receptor antagonist WAY100635 (0.5 mg/kg) was used. Results revealed that prolonged SSRIs attenuated AIMs expression and development in L-DOPA-primed and -naïve subjects, respectively, without interfering with motor performance. Neurochemical analysis of striatal tissue indicated that a 3 week SERT blockade increased DA levels in L-DOPA-treated rats. Pharmacologically, anti-dyskinetic effects were partially reversed with WAY100635 signifying involvement of the 5-HT1A receptor. Collectively, these findings demonstrate that prolonged SERT inhibition provides enduring anti-dyskinetic effects in part via 5-HT(1A) receptors while maintaining L-DOPA's anti-parkinsonian efficacy by enhancing striatal DA levels.

Multisite intracerebral microdialysis to study the mechanism of L-DOPA induced dopamine and serotonin release in the parkinsonian brain

L-DOPA is currently one of the best medications for Parkinson's disease. It was assumed for several years that its benefits and side effects were related to the enhancement of dopamine release in the dopamine-depleted striatum. The use of intracerebral microdialysis combined with a pharmacological approach has led to the discovery that serotonergic neurons are responsible for dopamine release induced by L-DOPA. The subsequent use of multisite microdialysis has further revealed that L-DOPA-stimulated dopamine release is widespread and related to the serotonergic innervation. The present Review emphasizes the functional impact of extrastriatal release of dopamine induced by L-DOPA in both the therapeutic and side effects of L-DOPA.

The monoamine re-uptake inhibitor UWA-101 improves motor fluctuations in the MPTP-lesioned common marmoset

BACKGROUND

The wearing-OFF phenomenon is a common motor complication of chronic L-3,4-dihydroxyphenylalanine (L-DOPA) therapy for Parkinson's disease. We recently described the discovery of UWA-101, a dual serotonin (SERT) and dopamine (DAT) transporter inhibitor, which increases the duration of "good quality" ON-time provided by L-DOPA in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned primate. Here, we further characterise the effects of UWA-101 on this extension of ON-time in terms of L-DOPA-induced side-effects in the MPTP-lesioned common marmoset.

METHODS

Marmosets were rendered parkinsonian by MPTP injection and "primed" by repeated L-DOPA administration, to exhibit dyskinesia and psychosis-like behaviours. Animals were then administered acute challenges of L-DOPA in combination with UWA-101 (1, 3, 6 and 10 mg/kg) or vehicle.

RESULTS

In combination with L-DOPA, UWA-101 (3, 6 and 10 mg/kg) significantly increased duration of ON-time (by 28%, 28%, and 33%, respectively; all P<0.05). UWA-101 (10 mg/kg) significantly extended duration of ON-time without disabling dyskinesia (by 62%, P<0.01). UWA-101 did not exacerbate the severity of dyskinesia (P>0.05). However, at the highest doses (6 and 10 mg/kg), UWA-101 increased the severity of psychosis-like behaviours (P<0.05).

CONCLUSIONS

Our results demonstrate that dual SERT/ DAT inhibitors can effectively enhance L-DOPA anti-parkinsonian action, without exacerbating dyskinesia and, as such, represent a promising new therapeutic class for wearing-OFF. However, at higher doses, dual SERT/ DAT inhibitors may exacerbate dopaminergic psychosis.

Serotonin transporter inhibition attenuates l-DOPA-induced dyskinesia without compromising l-DOPA efficacy in hemi-parkinsonian rats

Long-term dopamine replacement therapy with l-DOPA in Parkinson's disease often leads to the development of abnormal involuntary movements known as l-DOPA-induced dyskinesia. Growing evidence suggests that, following dopamine cell loss, serotonin neurons acting as surrogates for dopaminergic processes take up l-DOPA, convert it to dopamine and release it in an unregulated fashion that precipitates dyskinesia. Although most studies have focused on serotonin 5-HT(1) receptor stimulation as an antidyskinetic strategy, targeting the serotonin transporter modulation of dopamine activity has been overlooked. Therefore, in the current study, selective serotonin reuptake inhibitors were tested for their ability to reduce l-DOPA- and apomorphine-induced dyskinesia. In Experiments 1 and 2, hemi-parkinsonian rats were primed with l-DOPA until stable dyskinesia developed. Rats in Experiment 1 were administered the selective serotonin reuptake inhibitors paroxetine, citalopram or fluoxetine, followed by l-DOPA. Abnormal involuntary movements and forepaw adjusting steps were recorded to determine the effects of these compounds on dyskinesia and motor performance, respectively. Brains were collected on the final test day, after which striatal and raphe monoamines were examined via high-performance liquid chromatography. In Experiment 2, dyskinesias were measured after selective serotonin reuptake inhibitors and apomorphine. Serotonin reuptake inhibitors dose-dependently attenuated l-DOPA- but not apomorphine-induced dyskinesia, and preserved l-DOPA efficacy. Neurochemically, serotonin transporter inhibition enhanced striatal and raphe serotonin levels and reduced its turnover, indicating a potential mechanism of action. The present results support targeting serotonin transporters to improve Parkinson's disease treatment and provide further evidence for the role of the serotonin system in l-DOPA's effects.

Effect of selective serotonin reuptake inhibitors via 5-HT1A receptors on L-DOPA-induced rotational behavior in a hemiparkinsonian rat model

L-Dihydroxyphenylalanine (L-DOPA) is considered the gold standard for the treatment of Parkinson's disease (PD). However, long-term administration of L-DOPA can induce abnormal side effects. On the other hand, selective serotonin reuptake inhibitors (SSRIs) including fluoxetine have gained tremendous popularity in the treatment of depression in PD. SSRIs are thought to influence motor function in PD via pharmacological modification of interactions between serotonergic and dopaminergic networks, which are complex and not yet fully understand. In this study, intranigral injection of 6-hydroxydopamine (6-OHDA) in rats caused a significant loss of tyrosine hydroxylase immunoreactivity in the striatum and substantia nigra. However, tryptophan hydroxylase immunoreactivity of the striatum and raphe nucleus was unaffected by 6-OHDA. Immunohistochemical analysis reveal that the serotonergic system was unaffected by the injection of 6-OHDA. We demonstrated also that pre-treatment with fluoxetine significantly suppressed L-DOPA-induced rotational behavior. Additionally, fluoxetine suppressed L-DOPA-induced ERK1/2 and histone H3 phosphorylation. These effects of fluoxetine were abolished by pre-treatment with WAY 100135, a 5-HT(1A) antagonist. These results suggest that fluoxetine may influence motor function in PD via pharmacological modification of interactions between serotonergic and dopaminergic neuronal networks.

Imbalanced Dopaminergic Transmission Mediated by Serotonergic Neurons in L-DOPA-Induced Dyskinesia

L-DOPA-induced dyskinesias (LIDs) are one of the main motor side effects of L-DOPA therapy in Parkinson's disease. The review will consider the biochemical evidence indicating that the serotonergic neurons are involved in the dopaminergic effects of L-DOPA in the brain. The consequences are an ectopic and aberrant release of dopamine that follows the serotonergic innervation of the brain. After mid- to long-term treatment with L-DOPA, the pattern of L-DOPA-induced dopamine release is modified. In several brain regions, its effect is dramatically reduced while, in the striatum, its effect is quite preserved. LIDs could appear when the dopaminergic effects of L-DOPA fall in brain areas such as the cortex, enhancing the subcortical impact of dopamine and promoting aberrant motor responses. The consideration of the serotonergic system in the core mechanism of action of L-DOPA opens an important reserve of possible strategies to limit LIDs.

Serotonergic neurons mediate ectopic release of dopamine induced by L-DOPA in a rat model of Parkinson's disease

Benefit and motor side effects of l-DOPA in Parkinson's disease have been related to dopamine transmission in the striatum. However, the putative involvement of serotonergic neurons in the dopaminergic effects of l-DOPA suggests that the striatum is not a preferential target of l-DOPA. By using microdialysis in a rat model of Parkinson's disease, we found that l-DOPA (3-100 mg/kg) increased dopamine extracellular levels monitored simultaneously in four brain regions receiving serotonergic innervation: striatum, substantia nigra, hippocampus, prefrontal cortex. The increase was regionally similar at the lowest dose and 2-3 times stronger in the striatum at higher doses. Citalopram, a serotonin reuptake blocker, or the destruction of serotonergic fibers by 5,7-dihydroxytryptamine impaired l-DOPA-induced dopamine release in all regions. These data demonstrate that l-DOPA induces an ectopic release of dopamine due to serotonergic neurons. The new pattern of dopamine transmission created by l-DOPA may contribute to the benefit and side effects of l-DOPA.

The role of the dorsal raphe nucleus in the development, expression, and treatment of L-dopa-induced dyskinesia in hemiparkinsonian rats

Convergent evidence indicates that in later stages of Parkinson's disease raphestriatal serotonin neurons compensate for the loss of nigrostriatal dopamine neurons by converting and releasing dopamine derived from exogenous administration of the pharmacotherapeutic L-3,4-dihydroxyphenyl-L-alanine (L-dopa). Because the serotonin system is not equipped with dopamine autoregulatory mechanisms, it has been postulated that raphe-mediated striatal dopamine release may fluctuate dramatically. These fluctuations may portend the development of abnormal involuntary movements called L-dopa-induced dyskinesia (LID). As such, it has been hypothesized that reducing the activity of raphestriatal neurons could dampen supraphysiological stimulation of striatal dopamine receptors thereby alleviating LID. To directly address this, the current study employed the rodent model of LID to investigate the contribution of the rostral raphe nuclei (RRN) in the development, expression and treatment of LID. In the first study, dual serotonin/dopamine selective lesions of the RRN and medial forebrain bundle, respectively, verified that the RRN are essential for the development of LID. In a direct investigation into the neuroanatomical specificity of these effects, microinfusions of +/-8-OH-DPAT into the intact dorsal raphe nucleus dose-dependently attenuated the expression of LID without affecting the antiparkinsonian efficacy of L-dopa. These current findings reveal the integral contribution of the RRN in the development and expression of LID and implicate a prominent role for dorsal raphe 5-HT1AR in the efficacious properties of 5-HT1AR agonists.

In vivo microdialysis in Parkinson's research

Parkinson's disease (PD) is a progressive neurodegenerative disorder that is primarily characterized by the degeneration of dopamine (DA) neurons in the nigrostriatal system, which in turn produces profound neurochemical changes within the basal ganglia, representing the neural substrate for parkinsonian motor symptoms. The pathogenesis of the disease is still not completely understood, but environmental and genetic factors are thought to play important roles. Research into the pathogenesis and the development of new therapeutic intervention strategies that will slow or stop the progression of the disease in human has rapidly advanced by the use of neurotoxins that specifically target DA neurons. Over the years, a broad variety of experimental models of the disease has been developed and applied in diverse animal species. The two most common toxin models used employ 6-hydroxydopamine (6-OHDA) and the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine/1-methyl-4-phenilpyridinium ion (MPTP/MPP+), either given systemically or locally applied into the nigrostriatal pathway, to resemble PD features in animals. Both neurotoxins selectively and rapidly destroy catecolaminergic neurons, although with different mechanisms. Since in vivo microdialysis coupled to high-performance liquid chromatography is an established technique for studying physiological, pharmacological, and pathological changes of a wide range of low molecular weight substances in the brain extracellular fluid, here we review the most prominent animal and human data obtained by the use of this technique in PD research.

Reuptake of L-DOPA-derived extracellular DA in the striatum of a rodent model of Parkinson's disease via norepinephrine transporter

To determine the role of norepinephrine transporter in reuptake of L-DOPA-derived extracellular DA in the DA-denervated Parkinsonian striatum, we examined extracellular DA levels in the striatum of 6-hydroxyDA-lesioned rats that received L-DOPA (50 mg/kg with 12.5 mg/kg of benserazide) and L-DOPA plus desipramine (25 mg/kg), a selective norepinephrine reuptake inhibitor, using in vivo microdialysis. The pretreatment with desipramine increased levels of extracellular DA derived from administrated L-DOPA in the DA-denervated striatum. This study provides evidence that L-DOPA-derived DA is taken up by the norepinephrine transporter, instead of the dopamine transporter, in the striatum with dopaminergic denervation. This result suggests that the norepinephrine transporter could be a promising target in the treatment for Parkinson's disease.

Therapeutic potential of fluoxetine in neurological disorders

The selective serotonin reuptake inhibitor (SSRI) fluoxetine, which is registered for a variety of psychiatric disorders, has been found to stimulate the cAMP-responsive element binding protein (CREB), increase the production of brain-derived neurotrophic factor (BNDF) and the neurotrophic peptide S100beta, enhance glycogenolysis in astrocytes, block voltage-gated calcium and sodium channels, and decrease the conductance of mitochondrial voltage-dependent anion channels (VDACs). These mechanisms of actions suggest that fluoxetine may also have potential for the treatment of a number of neurological disorders. We performed a Pubmed search to review what is known about possible therapeutic effects of fluoxetine in animal models and patients with neurological disorders. Beneficial effects of fluoxetine have been noted in animal models of stroke, multiple sclerosis, and epilepsy. Fluoxetine was reported to improve neurological manifestations in patients with Alzheimer's disease, stroke, Huntington's disease, multiple sclerosis, traumatic brain injury, and epilepsy. Clinical studies so far were small and often poorly designed. Results were inconclusive and contradictory. However, the available preclinical data justify further clinical trials to determine the therapeutic potential of fluoxetine in neurological disorders.

Effects of coincident 5-HT1A receptor stimulation and NMDA receptor antagonism on L-DOPA-induced dyskinesia and rotational behaviors in the hemi-parkinsonian rat

RATIONALE

Serotonin 1A receptor (5-HT1AR) agonists reduce L-DOPA-induced dyskinesia and enhance motor function in experimental and clinical investigations of Parkinson's disease (PD). While the mechanism(s) by which these effects occur are unclear, recent research suggests that modulation of glutamate neurotransmission contributes.

OBJECTIVE

To further delineate the relationship between 5-HT1A receptors and glutamate, the current study examined the effects of the 5-HT1AR agonist, +/-8-OH-DPAT and the N-methyl-D-aspartic acid receptor (NMDAR) antagonist, MK-801, on L-DOPA-induced motor behavior.

MATERIALS AND METHODS

Unilateral 6-hydroxydopamine lesioned male Sprague-Dawley rats were rendered dyskinetic with 1 week of daily L-DOPA (12 mg/kg, i.p.) + benserazide (15 mg/kg, i.p.). On test days, one group of rats received pretreatments of: +/-8-OH-DPAT (0, 0.03, 0.1, 0.3 mg/kg, i.p.) or MK-801 (0, 0.03, 0.1, 0.3 mg/kg, i.p.). A second group was administered combined +/-8-OH-DPAT (0, 0.03 or 0.1 mg/kg, i.p.) + MK-801 (0, 0.1 mg/kg, i.p.). Pretreatments were followed by L-DOPA administration, after which, abnormal involuntary movements (AIMs) and rotations were monitored. To investigate effects on motor performance, subthreshold doses of +/-8-OH-DPAT (0.03 mg/kg, i.p.) + MK-801 (0.1 mg/kg, i.p.) were administered to L-DOPA-naïve hemiparkinsonian rats before the forepaw adjusting steps test.

RESULTS

Individually, both +/-8-OH-DPAT and MK-801 dose-dependently decreased L-DOPA-induced AIMs without affecting rotations. Combined subthreshold doses of +/-8-OH-DPAT+MK-801 reduced L-DOPA-induced AIMs and potently enhanced contralateral rotations without altering L-DOPA-induced motor improvements.

CONCLUSIONS

The current results indicate a functional interaction between 5-HT1AR and NMDAR that may improve pharmacological treatment of PD patients.

Depressive disorders and anti-parkinson drug treatment: a case register study

OBJECTIVE

To estimate the rate of treatment with anti-parkinson drugs (APD) among patients with depression.

METHOD

In a nationwide case register linkage study, all persons with a main diagnosis of depression during 5 years were identified. A control group of persons with diagnoses of osteoarthritis was included. The subsequent risk of getting treatment with APD was estimated for the two groups. Statistical analyses involved Poisson's regression and competing risk models.

RESULTS

A total of 14 991 persons were included. The rate of getting APD was 2.57 (95% CI: 1.46-4.52) times higher for persons with depression than for persons with osteoarthritis. Overall, the rate was highest for men. However, women with depression had a 3.89 (95% CI: 1.98-7.62) times higher rate of APD treatment as women with osteoarthritis while no significant difference was found among men.

CONCLUSION

Provided that prescription of APD reflects the presence of Parkinson's disease, results support a positive statistical association between depressive disorders and Parkinson's disease.

In 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine-Treated Primates, the Selective 5-Hydroxytryptamine 1a Agonist (R)-(+)-8-OHDPAT Inhibits Levodopa-Induced Dyskinesia but Only with\ Increased Motor Disability

5-Hydroxytryptamine 1a (5-HT(1a)) receptor agonists, such as sarizotan and tandospirone, are reported to reduce levodopa-induced dyskinesia in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated macaques and in Parkinson's disease without worsening motor disability. However, these compounds are not specific for 5-HT(1a) receptors and also possess dopamine antagonist actions. We now report on the effects of (2R)-(+)-8-hydroxy-2-(di-n-propylamino)tetralin [(R)-(+)-8-OHDPAT], a selective 5-HT(1a) agonist lacking dopaminergic activity, on motor disability and dyskinesia (chorea and dystonia) in levodopa-primed MPTP-treated common marmosets. Administration of (R)-(+)-8-OHDPAT (0.2, 0.6, and 2.0 mg/kg s.c), in conjunction with levodopa/carbidopa (12.5 mg/kg each p.o.) to levodopa-primed animals, dose-dependently reduced levodopa-induced chorea but did not affect dystonic movements. However, (R)-(+)-8-OHDPAT treatment also reduced locomotor activity and the reversal of motor disability. Administration of (R)-(+)-8-OHDPAT alone had no effects of motor behaviors. The effects of (R)-(+)-8-OHDPAT on levodopa-induced motor behaviors were antagonized by the 5-HT(1a) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide maleate (WAY-100635) (1.0 mg/kg s.c.). Administration of (R)-(+)-8-OHDPAT (0.6 mg/kg s.c.) also reduced chorea produced by the administration of the D(2)/D(3) dopamine receptor agonist pramipexole (0.06 mg/kg p.o.) to levodopa-primed MPTP-treated animals. However, again the increase in locomotor activity and reversal of motor disability produced by pramipexole were also inhibited. These data suggest that selective 5-HT(1a) agonists do not provide an effective means of suppressing levodopa-induced dyskinesia, except with worsening of parkinsonism.

MDMA and fenfluramine reduce L-DOPA-induced dyskinesia via indirect 5-HT1A receptor stimulation

Chronic L-3,4-dihydroxyphenylalanine (L-DOPA) pharmacotherapy in Parkinson's disease is often accompanied by the development of abnormal and excessive movements known as dyskinesia. Clinical and experimental studies indicate that indirect serotonin agonists can suppress dyskinesia without affecting the efficacy of L-DOPA. While the mechanism by which these effects occur is not clear, recent research suggests that serotonin 5-HT1A receptors may play a pivotal role. To test this, male Sprague-Dawley rats with unilateral 6-hydroxydopamine medial forebrain bundle lesions received 1 week of daily treatment with L-DOPA (12 mg/kg, i.p.) plus benserazide (15 mg/kg, i.p.). Beginning on the 8th day of treatment and every 3rd or 4th day thereafter, rats were pretreated with vehicle (0.9% NaCl), the serotonin and dopamine releaser 3,4-methylenedioxymethamphetamine (MDMA; 0.25 or 2.5 mg/kg, i.p.) or the serotonin releaser fenfluramine (FEN; 0.25 or 2.5 mg/kg, i.p.) 5 min prior to L-DOPA, after which abnormal involuntary movements (AIMs) and rotations were quantified every 20th minute for 2 h. Pretreatment with 2.5 mg/kg of either MDMA or FEN reduced AIMs. To determine the contribution of the 5-HT1A receptor to these effects, another group of L-DOPA-primed 6-hydroxydopamine-lesioned rats were pretreated with the 5-HT1A antagonist WAY100635 (0.5 mg/kg, i.p.), MDMA + WAY100635 (2.5 + 0.5 mg/kg, i.p.) or FEN + WAY100635 (2.5 + 0.5 mg/kg, i.p.) 5 min prior to L-DOPA and subsequent AIMs and rotation tests. The antidyskinetic effects of MDMA and FEN were reversed by cotreatment with WAY100635. These results suggest that 5-HT-augmenting compounds such as MDMA and FEN probably convey antidyskinetic properties in part via stimulation of 5-HT1A receptors.

Reuptake of l-DOPA-derived extracellular dopamine in the striatum with dopaminergic denervation via serotonin transporters

The aim of our present study was to determine whether exogenous L-DOPA-derived extracellular dopamine (DA) in the denervated striatum is taken up via serotonin (5-HT) transporters. Rats with dopaminergic denervation by 6-hydroxydopamine were injected with L-DOPA 50 mg/kg (i.p.) and extracellular DA levels in the denervated striatum were measured using in vivo microdialysis technique. During the experiments, fluoxetine, a selective serotonin reuptake inhibitor, was locally perfused into the striatum through the microdialysis probe. Cumulative amounts of L-DOPA-derived extracellular DA during 300 min were increased by fluoxetine dose-dependently (10 microM and 50 microM). These results suggest that reuptake of DA via 5-HT transporters plays an important role in the clearance of L-DOPA-derived DA from the extracellular space when dopaminergic neurons are extensively denervated.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

L-dopa impairs learning, but spares generalization, in Parkinson's disease

In this study we examined the effect of dopaminergic modulation on learning and memory. Parkinson's patients were tested 'on' versus 'off' dopaminergic medication, using a two-phase learning and transfer task. We found that dopaminergic medication was associated with impaired learning of an incrementally acquired concurrent discrimination task, while patients withdrawn from dopaminergic medication performed as well as controls. In addition, we found a dissociation of the effect of medication within a single two-phase task: patients tested 'on' medication were not impaired at the ability to generalize based on learned information. The deficit among medicated patients appeared to be related specifically to the concurrent, incremental, feedback-based nature of the task: such a deficit was not found in a version of the task in which demands for concurrent error-processing learning were reduced. Taken together with a growing body of evidence emphasizing a role for midbrain dopamine in error-correcting, feedback-based learning processes, the present results suggest a framework for understanding previously conflicting results regarding the effect of medication on learning and memory in Parkinson's disease.

Differential effects of reboxetine and citalopram on hand-motor function in patients suffering from major depression

RATIONALE

Motor dysfunctions might be a more common side effect of serotonergic than noradrenergic antidepressants. However, the effects of antidepressants on motor function in depression have rarely been analyzed systematically. Computerized methods allow the objective registration of drug-induced motor dysfunction and were applied in this study.

OBJECTIVES

To examine the effects of a selective noradrenaline re-uptake inhibitor (NARI) (reboxetine) and a selective serotonin re-uptake inhibitor (SSRI) (citalopram) on hand-motor function in patients with major depression.

METHODS

Different types of hand movements (drawing of circles and handwriting probes) were recorded and analyzed in 16 acutely depressed inpatients receiving citalopram (30-60 mg/day) and 12 acutely depressed inpatients treated with reboxetine (4-8 mg/day), using a digitizing tablet for the analysis of movement dynamics. Both groups were comparable regarding mean age (42-43 years), gender, handedness (preponderance of right-handers) and the mean baseline HAMD score (about 27). Five kinematical parameters reflecting velocity, regularity and degree of automation of hand movements have been computed.

RESULTS

Reboxetine had significantly more favorable effects on fine motor function (increased velocity of rapid hand movements) in depressed patients than citalopram. These differences became obvious when patients conducted more complex tasks and are not explained by differential antidepressant effects.

CONCLUSIONS

Our findings are in line with the hypothesis that SSRI tend to have small, but more pronounced negative effects on motor function than NARI.

The role of dopamine in cognitive sequence learning: evidence from Parkinson’s disease

Electrophysiological and computational studies suggest that nigro-striatal dopamine may play an important role in learning about sequences of environmentally important stimuli, particularly when this learning is based upon step-by-step associations between stimuli, such as in second-order conditioning. If so, one would predict that disruption of the midbrain dopamine system--such as occurs in Parkinson's disease--may lead to deficits on tasks that rely upon such learning processes. This hypothesis was tested using a "chaining" task, in which each additional link in a sequence of stimuli leading to reward is trained step-by-step, until a full sequence is learned. We further examined how medication (L-dopa) affects this type of learning. As predicted, we found that Parkinson's patients tested 'off' L-dopa performed as well as controls during the first phase of this task, when required to learn a simple stimulus-response association, but were impaired at learning the full sequence of stimuli. In contrast, we found that Parkinson's patients tested 'on' L-dopa performed better than those tested 'off', and no worse than controls, on all phases of the task. These findings suggest that the loss of dopamine that occurs in Parkinson's disease can lead to specific learning impairments that are predicted by electrophysiological and computational studies, and that enhancing dopamine levels with L-dopa alleviates this deficit. This last result raises questions regarding the mechanisms by which midbrain dopamine modulates learning in Parkinson's disease, and how L-dopa affects these processes.

D2 dopamine receptor gene polymorphism: paroxetine and social functioning in posttraumatic stress disorder

This study examined whether allelic status of the D2 dopamine receptor (DRD2) gene was associated with response to a selective serotonin reuptake inhibitor, paroxetine, in the treatment of posttraumatic stress disorder (PTSD). Sixty-three Caucasian war veterans with combat-related PTSD were treated with paroxetine for 8 weeks. Patients were assessed at baseline and at follow-up using the General Health Questionnaire-28 (GHQ). TaqI A DRD2 alleles were determined by PCR. Before paroxetine treatment, patients with the DRD2 A1+ allele (A1A2 genotype) compared to those with the A1- allele (A2A2 genotype) had higher total GHQ psychopathological scores (P=0.040) and higher GHQ subscale scores for anxiety/insomnia (0.046), social dysfunction (P=0.033) and depression (P=0.011). In an intention-to-treat analysis, paroxetine was associated with significant improvement in total GHQ scores (P=0.014) and in the factor scores of social dysfunction (P=0.033), anxiety (P=0.009) and depression (P=0.026). Furthermore, there was a significant allele by time interaction on the social dysfunction scale, with A1+ allelic patients showing significant improvement in social functioning compared to A1- allelic patients (P=0.031), an effect independent of changes in depression or anxiety. This suggests changes in social functioning induced by paroxetine may be, in part, mediated via D2 dopamine receptors. The DRD2 A1 allele may prove to be a useful marker to assist clinicians in predicting which patients with PTSD are likely to obtain improvements in social functioning with paroxetine treatment.

Rapid induction of serotonergic hyperinnervation in the adult rat striatum with extensive dopaminergic denervation

The aim of our study was to determine whether serotonergic hyperinnervation is rapidly induced in the striatum of adult rats with extensive dopaminergic denervation. Immunohistochemical study was performed on the brain sections obtained at 2 and 8 weeks after injection of 6-hydroxydopamine (6-OHDA) into the medial forebrain bundle. The extent of dopaminergic denervation was evaluated as a percentage loss of tyrosine hydroxylase immunopositive neurons in the substantia nigra pars compacta. The immunopositive areas for serotonin (5-HT) in the striatum were measured. In the lesioned rats 97.5+/-0.6% of dopamine neurons were lost. 5-HT immunopositive areas in the striatum were significantly increased both at 2 and 8 weeks after 6-OHDA injection (P<0.01). These results suggest that extensive dopaminergic denervation in adult rats induces rapid serotonergic hyperinnervation in the striatum as early as 2 weeks after lesioning.

Effects of benserazide on L-DOPA-derived extracellular dopamine levels and aromatic L-amino acid decarboxylase activity in the striatum of 6-hydroxydopamine-lesioned rats

Benserazide is commonly used for Parkinson's disease in combination with L-DOPA as a peripheral aromatic L-amino acid decarboxylase (AADC) inhibitor. However, recent studies using intact animals indicate that benserazide acts also in the central nervous system. We determined the influence of benserazide on the central AADC activity in rats with dopaminergic denervation and observed changes in extracellular dopamine (DA) levels after benserazide and L-DOPA administration. First, using in vivo microdialysis technique, we measured extracellular DA levels in the striatum of 6-hydroxydopamine (6-OHDA)-lesioned rats treated with benserazide and L-DOPA. Second, we measured AADC activity in the striatal tissues after benserazide administration. Although administration of 5, 10 and 50 mg/kg benserazide to 6-OHDA-lesioned rats showed an identical increase in exogenous L-DOPA-derived extracellular DA levels, the time to reach the peak DA levels were significantly prolonged by benserazide dose-dependently. The AADC activity in the denervated striatal tissues showed a significant decrease by 10 mg/kg and 50 mg/kg benserazide. These results suggest that benserazide reduces the central AADC activity in the striatum of rats with nigrostriatal denervation, which leads to changes in the metabolism of exogenous L-DOPA. Central activity of AADC inhibitors should be taken into consideration when they are used both in experimental and clinical studies on Parkinson's disease.

Dopamine, but not norepinephrine or serotonin, reuptake inhibition reverses motor deficits in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated primates

Monoamine reuptake inhibitors that do not discriminate between the transporters for dopamine (DA), norepinephrine (NE), or 5-hydroxytryptamine (5-HT, serotonin) can reverse locomotor deficits and motor disability in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated common marmosets. DA reuptake inhibition is presumed to be primarily responsible, but the role played by inhibition of NE and 5-HT reuptake is unknown. We now evaluate the efficacy of a range of monoamine reuptake inhibitors either alone or in combination in MPTP-treated common marmosets to determine the actions required for effective antiparkinsonian activity. Monoamine reuptake inhibitors not discriminating between the DA, NE, and 5-HT transporters [1-[1-(3,4-dichlororphenyl)cyclobutyl]-2-(3-diaminethylaminopropylthio)ethanone monocitrate (BTS 74 398) and nomifensine] reversed locomotor deficits and motor disability in MPTP-treated marmosets but bupropion was without effect. The selective DA reuptake inhibitor 1-(2-(bis-(4-fluorophenyl)-methoxy)ethyl)-4-(3-phenylpropyl) piperazine) dihydrochloride (GBR 12909) also reversed these motor deficits. The relative efficacy of the compounds (BTS 74 398 > GBR 12909 > nomifensine >> bupropion) paralleled their potency in inhibiting DA uptake in vitro and in vivo. In contrast, the selective NE reuptake inhibitor nisoxetine and the 5-HT reuptake inhibitor sertraline administered alone failed to improve motor function and tended to worsen the deficits. Coadministration of nisoxetine attenuated the improvement in motor deficits produced by GBR 12909. Coadministration of sertraline also abolished the reversal of motor deficits produced by GBR 12909. Coadministration of both sertraline and nisoxetine similarly abolished the improvement of motor deficits produced by GBR 12909. Molecules possessing potent DA reuptake inhibitory activity may be useful in the treatment of the motor symptoms of Parkinson's disease. In contrast, there seems to be no role for NE or 5-HT reuptake inhibitors, and they may impair antiparkinsonian activity mediated through dopaminergic mechanisms.