Genetic basis of susceptibility to low-dose paraquat and variation between the sexes in Drosophila melanogaster.

Toxicant resistance is a complex trait, affected both by genetics and the environment. Like most complex traits, it can exhibit sexual dimorphism, yet sex is often overlooked as a factor in studies of toxicant resistance. Paraquat, one such toxicant, is a commonly used herbicide and is known to produce mitochondrial oxidative stress, decrease dopaminergic neurons and dopamine (DA) levels, and decrease motor ability. While the main effects of paraquat are well-characterized, less is known about the naturally occurring variation in paraquat susceptibility. The purpose of this study was to map the genes contributing to low-dose paraquat susceptibility in Drosophila melanogaster, and to determine if susceptibility differs between the sexes. One hundred of the Drosophila Genetic Reference Panel (DGRP) lines were scored for susceptibility via climbing ability and used in a genome-wide association study (GWAS). Variation in seventeen genes in females and thirty-five genes in males associated with paraquat susceptibility. Only two candidate genes overlapped between the sexes despite a significant positive correlation between male and female susceptibilities. Many associated polymorphisms had significant interactions with sex, with most having conditionally neutral effects. Conditional neutrality between the sexes probably stems from sex-biased expression which may result from partial resolution of sexual conflict. Candidate genes were verified with RNAi knockdowns, gene expression analyses, and DA quantification. Several of these genes are novel associations with paraquat susceptibility. This research highlights the importance of assessing both sexes when studying toxicant susceptibility.

Striatal Nurr1, but not FosB expression links a levodopa-induced dyskinesia phenotype to genotype in Fisher 344 vs. Lewis hemiparkinsonian rats.

Numerous genes, and alterations in their expression, have been identified as risk factors for developing levodopa-induced dyskinesia (LID). However, our understanding of the complexities of molecular changes remains insufficient for development of clinical treatment. In the current study we used gene array, in situ hybridization, immunohistochemistry, and microdialysis to provide a unique compare and contrast assessment of the relationship of four candidate genes to LID, employing three genetically distinct rat strains (Sprague-Dawley (SD), Fischer-344 (F344) and Lewis-RT.1) showing differences in dyskinesia susceptibility and 'first-ever LID' versus 'chronic LID' expression in subjects displaying equal dyskinesia severity. In these studies, rat strains were easily distinguishable for their LID propensity with: 1) a majority of SD rats expressing LID (LID+) and a subset being resistant (LID-); 2) all F344 rats readily developing (LID+); and 3) all Lewis rats being LID-resistant (LID-). Following chronic levodopa, LID+ SD rats showed significant increases in candidate gene expression: Nr4a2/(Nurr1) > > Trh > Inhba = Fosb. However, SD rats with long-standing striatal dopamine (DA) depletion treated with first-ever versus chronic high-dose levodopa revealed that despite identical levels of LID severity: 1) Fosb and Nurr1 transcripts but not protein were elevated with acute LID expression; 2) FOSB/ΔFOSB and NURR1 proteins were elevated only with chronic LID; and 3) Trh transcript and protein were elevated only with chronic LID. Strikingly, despite similar levodopa-induced striatal DA release in both LID-expressing F344 and LID-resistant Lewis rats, Fosb, Trh, Inhba transcripts were significantly elevated in both strains; however, Nurr1 mRNA was significantly increased only in LID+ F344 rats. These findings suggest a need to reevaluate currently accepted genotype-to-phenotype relationships in the expression of LID, specifically that of Fosb, a transcription factor generally assumed to play a causal role, and Nurr1, a transcription factor that has received significant attention in PD research linked to its critical role in the survival and function of midbrain DA neurons but who's striatal expression, generally below levels of detection, has remained largely unexplored as a regulator of LID. Finally these studies introduce a novel 'model' (inbred F344 vs inbred Lewis) that may provide a powerful tool for investigating the role for 'dyskinesia-resistance' genes downstream of 'dyskinesia-susceptibility' genes in modulating LID expression, a concept that has received considerably less attention and offers a new ways of thinking about antidyskinetic therapies.

Effects of Muscarinic Acetylcholine m1 and m4 Receptor Blockade on Dyskinesia in the Hemi-Parkinsonian Rat.

Standard treatment for Parkinson's disease (PD) is L-DOPA, but with chronic administration the majority of patients develop L-DOPA-induced dyskinesia (LID). Emerging evidence implicates the cholinergic system in PD and LID. Muscarinic acetylcholine receptors (mAChR) are known to modulate movement and of late have been implicated as possible targets for LID. Therefore the current study investigated the role of M1 and M4 mAChRs in LID, on motor performance following L-DOPA treatment, and sought to identify brain sites through which these receptors were acting. We first administered M1R-preferring antagonist trihexyphenidyl (0, 0.1, and 1.0 mg/kg, i.p.) or the M4R-preferring antagonist tropicamide (0, 10, and 30 mg/kg, i.p.) before L-DOPA, after which LID and motor performance were evaluated. Both compounds worsened and extended the time course of LID, while M1R blockade improved motor performance. We then evaluated the effects of tropicamide and trihexyphenidyl on dyskinesia induced by D1R agonist SKF81297 or D2R agonist quinpirole. Surprisingly, both M1R and M4R antagonists reduced D1R agonist-induced dyskinesia but not D2R agonist-induced dyskinesia, suggesting that mAChR blockade differentially affects MSN firing in the absence of postsynaptic DA. Finally, we evaluated effects of striatum- or PPN-targeted tropicamide microinfusion on LID and motor performance. Despite prior evidence, M4R blockade in either site alone did not affect the severity of LID via local striatal or PPN infusions. Taken together, these data suggest M4R as a promising therapeutic target for reducing LID using more selective compounds.

Diverse serotonin actions of vilazodone reduce l-3,4-dihidroxyphenylalanine-induced dyskinesia in hemi-parkinsonian rats.

BACKGROUND: The serotonergic system is a well-established modulator of l-dopa-induced dyskinesia. To date, targeting serotonin transporters or serotonin receptor subtype 1A (5-HT1A ) reduces l-dopa-induced dyskinesia in animal models; however, these strategies have failed to translate clinically. Ideally, a compound acting at both known antidyskinetic sites could optimize serotonin-mediated approaches. Vilazodone is a selective serotonin reuptake inhibitor and a partial 5-HT1A agonist approved by the U.S. Food and Drug Administration, situating Vilazodone in a unique position to reduce l-dopa-induced dyskinesia without compromising l-dopa-mediated motor improvements. OBJECTIVES: The goal of the present study was to characterize Vilazodone's effects on l-dopa-induced behaviors, neurochemistry and gene expression in unilateral 6-hydroxydopamine-lesioned hemi-parkinsonian rats. METHODS: In experiments 1 and 2, l-dopa-naïve and l-dopa-primed animals were coadministered Vilazodone and l-dopa daily for 3 weeks to model subchronic use, and behavioral, neurochemical, and messenger RNA (mRNA) expression changes were measured. In experiment 3, dyskinetic behavior was assessed following 5-HT1A or serotonin receptor subtype 1B blockade prior to Vilazodone-l-dopa coadministration. RESULTS: Vilazodone significantly suppressed developing and established l-dopa-induced dyskinesia without compromising the promotor effects of l-dopa therapy. In the dopamine-depleted striatum, Vilazodone-l-dopa cotreatment increased dopamine content, suggesting a normalization of dopamine kinetics in dyskinetic brain, and reduced l-dopa-induced c-Fos and preprodynorphin mRNA overexpression, indicative of attenuated dopamine D1 receptor-mediated direct pathway overactivity. Only 5-HT1A antagonism partially attenuated Vilazodone's antidyskinetic efficacy, suggesting both serotonin transporter-dependent effects and 5-HT1A receptors in Vilazodone's actions. CONCLUSIONS: Our findings show Vilazodone has a serotonin-dependent effect on rodent l-dopa-induced dyskinesia and implicate the potential for repositioning Vilazodone against l-dopa-induced dyskinesia development and expression in Parkinson's disease patients. © 2018 International Parkinson and Movement Disorder Society.

Reduction of PINK1 or DJ-1 impair mitochondrial motility in neurites and alter ER-mitochondria contacts.

Subcellular distribution of mitochondria in neurons is crucial for meeting the energetic demands, as well as the necessity to buffer Ca2+ within the axon, dendrites and synapses. Mitochondrial impairment is an important feature of Parkinson disease (PD), in which both familial parkinsonism genes DJ-1 and PINK1 have a great impact on mitochondrial function. We used differentiated human dopaminergic neuroblastoma cell lines with stable PINK1 or DJ-1 knockdown to study live motility of mitochondria in neurites. The frequency of anterograde and retrograde mitochondrial motility was decreased in PINK1 knockdown cells and the frequency of total mitochondrial motility events was reduced in both cell lines. However, neither the distribution nor the size of mitochondria in the neurites differed from the control cells even after downregulation of the mitochondrial fission protein, Drp1. Furthermore, mitochondria from PINK1 knockdown cells, in which motility was most impaired, had increased levels of GSK3ßSer9 and higher release of mitochondrial Ca2+ when exposed to CCCP-induced mitochondrial uncoupling. Further analysis of the ER-mitochondria contacts involved in Ca2+ shuttling showed that PINK1 knockdown cells had reduced contacts between the two organelles. Our results give new insight on how PINK1 and DJ-1 influence mitochondria, thus providing clues to novel PD therapies.

Detection of endogenous S1292 LRRK2 autophosphorylation in mouse tissue as a readout for kinase activity.

Parkinson's disease-linked mutations in LRRK2 enhance the kinase activity of the protein, therefore targeting LRRK2 kinase activity is a promising therapeutic approach. Phosphorylation at S935 of LRRK2 and of its Rab GTPase substrates have proven very useful biomarkers to monitor its kinase activity. Complementary to these approaches autophosphorylation of LRRK2 can be used as a direct kinase activity readout but to date detection of autophosphorylation at endogenous levels in vivo has been limited. We developed a fractionation-based enrichment method to successfully detect endogenous S1292 LRRK2 autophosphorylation in mouse tissues and highlight S1292 as a physiological readout candidate for LRRK2 kinase activity in vivo.

A new outlook on cholinergic interneurons in Parkinson's disease and L-DOPA-induced dyskinesia.

Traditionally, dopamine (DA) and acetylcholine (ACh) striatal systems were considered antagonistic and imbalances or aberrant signaling between these neurotransmitter systems could be detrimental to basal ganglia activity and pursuant motor function, such as in Parkinson's disease (PD) and L-DOPA-induced dyskinesia (LID). Herein, we discuss the involvement of cholinergic interneurons (ChIs) in striatally-mediated movement in a healthy, parkinsonian, and dyskinetic state. ChIs integrate numerous neurotransmitter signals using intrinsic glutamate, serotonin, and DA receptors and convey the appropriate transmission onto nearby muscarinic and nicotinic ACh receptors to produce movement. In PD, severe DA depletion causes abnormal rises in ChI activity which promote striatal signaling to attenuate normal movement. When treating PD with L-DOPA, hyperkinetic side effects, or LID, develop due to increased striatal DA; however, the role of ChIs and ACh transmission, until recently has been unclear. Fortunately, new technology and pharmacological agents have facilitated understanding of ChI function and ACh signaling in the context of LID, thus offering new opportunities to modify existing and discover future therapeutic strategies in movement disorders.

Hexokinases link DJ-1 to the PINK1/parkin pathway.

BACKGROUND: Early onset Parkinson's disease is caused by variants in PINK1, parkin, and DJ-1. PINK1 and parkin operate in pathways that preserve mitochondrial integrity, but the function of DJ-1 and how it relates to PINK1 and parkin is poorly understood. METHODS: A series of unbiased high-content screens were used to analyze changes at the protein, RNA, and metabolite level in rodent brains lacking DJ-1. Results were validated using targeted approaches, and cellular assays were performed to probe the mechanisms involved. RESULTS: We find that in both rat and mouse brains, DJ-1 knockout results in an age-dependent accumulation of hexokinase 1 in the cytosol, away from its usual location at the mitochondria, with subsequent activation of the polyol pathway of glucose metabolism in vivo. Both in the brain and in cultured cells, DJ-1 deficiency is associated with accumulation of the phosphatase PTEN that antagonizes the kinase AKT. In cells, addition of an inhibitor of AKT (MK2206) or addition of a peptide to dissociate association of hexokinases from mitochondria both inhibit the PINK1/parkin pathway, which works to maintain mitochondrial integrity. CONCLUSION: Hexokinases are an important link between three major genetic causes of early onset Parkinson's disease. Because aging is associated with deregulated nutrient sensing, these results help explain why DJ-1 is associated with age-dependent disease.

D-512, a novel dopamine D2/3 receptor agonist, demonstrates greater anti-Parkinsonian efficacy than ropinirole in Parkinsonian rats.

BACKGROUND AND PURPOSE: Symptoms of Parkinson's disease are commonly managed using selective dopamine D2/3 receptor agonists, including ropinirole. While D2/3 agonists are useful in early-stage Parkinson's disease, they tend to lose efficacy in later disease stages and do not appear to modify disease progression. We have recently developed a novel 'multifunctional' compound, D-512: a high-affinity D2/3 receptor agonist with antioxidant and other neuroprotective properties that may limit Parkinson's disease progression. This study sought to compare the anti-Parkinsonian properties of the clinically used compound, ropinirole, with those of the novel compound, D-512. EXPERIMENTAL APPROACH: A rat model of Parkinson's disease was created by unilaterally infusing 6-hydroxydopamine, a dopamine neurotoxin, into the medial forebrain bundle. D-512 was compared with ropinirole for ability to stimulate spontaneous motor activity and reverse Parkinsonian akinesia. These beneficial effects were compared against each drug's liability to provoke dyskinesia, a common motor side effect. KEY RESULTS: Both compounds increased spontaneous movement, but D-512 showed a longer duration of action. Only D-512 was able to significantly reverse forelimb akinesia. Drug-induced dyskinesia was similar for equivalent doses. CONCLUSIONS AND IMPLICATIONS: Compared with ropinirole, D-512 showed greater peak-dose efficacy and a longer duration of action, despite a similar side-effect profile. Our results add to earlier data showing that D-512 is superior to available D2/3 agonists and could merit clinical investigation.

Characterizing the differential roles of striatal 5-HT1A auto- and hetero-receptors in the reduction of l-DOPA-induced dyskinesia.

l-DOPA remains the benchmark treatment for Parkinson's disease (PD) motor symptoms, but chronic use leads to l-DOPA-induced dyskinesia (LID). The serotonin (5-HT) system has been established as a key modulator of LID and 5-HT1A receptors (5-HT1AR) stimulation has been shown to convey anti-dyskinetic effects. However, 5-HT1AR agonists often compromise clinical efficacy or display intrinsic side effects and their site(s) of actions remain debatable. Recently, highly selective G-protein biased 5-HT1AR agonists, F13714 and F15599, were shown to potently target 5-HT1A auto- or hetero-receptors, respectively. The current investigation sought to identify the signaling mechanisms and neuroanatomical substrates by which 5-HT1AR produce behavioral effects. In experiment 1, hemi-parkinsonian, l-DOPA-primed rats received systemic injections of vehicle, F13714 (0.01 or 0.02mg/kg), or F15599 (0.06 or 0.12mg/kg) 5min prior to l-DOPA (6mg/kg), after which LID, motor performance and 5-HT syndrome were rated. Both compounds significantly reduced LID, without affecting motor performance, however, acute administration of F13714 significantly induced 5-HT syndrome at anti-dyskinetic doses. In experiment 2, we elucidated the role of striatal 5-HT1AR in the effects of F13714 and F15599. Hemi-parkinsonian, l-DOPA-primed rats received bilateral intra-striatal microinjections of either F13714 (0, 2 or 10µg/side) or F15599 (0, 10 or 30µg/side) 5min prior to systemic l-DOPA (6mg/kg). Intra-striatal effects mimicked systemic effects, suggesting that striatal 5-HT1AR sub-populations play an important role in the anti-LID and pro-5-HT syndrome profiles of F13714 and F15599. Finally, in experiment 3, we examined the effects of F13714 and F15599 on D1 receptor (D1R) agonist-induced dyskinesia by administering either compound 5min prior to SKF 38393 (2mg/kg). While F13714 resulted in a mild delay in D1R-mediated dyskinesia, F15599 had no effect. Collectively these data suggest that the F-series compounds articulate their anti-LID effects through activation of a diverse set of striatal 5-HT1A hetero-receptor populations.

The Role of Primary Motor Cortex (M1) Glutamate and GABA Signaling in l-DOPA-Induced Dyskinesia in Parkinsonian Rats.

UNLABELLED: Long-term treatment of Parkinson's disease with l-DOPA almost always leads to the development of involuntary movements termed l-DOPA-induced dyskinesia. Whereas hyperdopaminergic signaling in the basal ganglia is thought to cause dyskinesia, alterations in primary motor cortex (M1) activity are also prominent during dyskinesia, suggesting that the cortex may represent a therapeutic target. The present study used the rat unilateral 6-hydroxydopamine lesion model of Parkinson's disease to characterize in vivo changes in GABA and glutamate neurotransmission within M1 and determine their contribution to behavioral output. 6-Hydroxydopamine lesion led to parkinsonian motor impairment that was partially reversed by l-DOPA. Among sham-lesioned rats, l-DOPA did not change glutamate or GABA efflux. Likewise, 6-hydroxydopamine lesion did not impact GABA or glutamate among rats chronically treated with saline. However, we observed an interaction of lesion and treatment whereby, among lesioned rats, l-DOPA given acutely (1 d) or chronically (14-16 d) reduced glutamate efflux and enhanced GABA efflux. Site-specific microinjections into M1 demonstrated that l-DOPA-induced dyskinesia was reduced by M1 infusion of a D1 antagonist, an AMPA antagonist, or a GABAA agonist. Overall, the present study demonstrates that l-DOPA-induced dyskinesia is associated with increased M1 inhibition and that exogenously enhancing M1 inhibition may attenuate dyskinesia, findings that are in agreement with functional imaging and transcranial magnetic stimulation studies in human Parkinson's disease patients. Together, our study suggests that increasing M1 inhibitory tone is an endogenous compensatory response designed to limit dyskinesia severity and that potentiating this response is a viable therapeutic strategy. SIGNIFICANCE STATEMENT: Most Parkinson's disease patients will receive l-DOPA and eventually develop hyperkinetic involuntary movements termed dyskinesia. Such symptoms can be as debilitating as the disease itself. Although dyskinesia is associated with dynamic changes in primary motor cortex physiology, to date, there are no published studies investigating in vivo neurotransmitter release in M1 during dyskinesia. In parkinsonian rats, l-DOPA administration reduced M1 glutamate efflux and enhanced GABA efflux, coincident with the emergence of dyskinetic behaviors. Dyskinesia could be reduced by local M1 modulation of D1, AMPA, and GABAA receptors, providing preclinical support for the notion that exogenously blunting M1 signaling (pharmacologically or with cortical stimulation) is a therapeutic approach to the treatment of debilitating dyskinesias.

Monoamine transporter contributions to l-DOPA effects in hemi-parkinsonian rats.

l-DOPA is the standard treatment for Parkinson's disease (PD), but chronic treatment typically leads to abnormal involuntary movement or dyskinesia (LID) development. Although poorly understood, dyskinetic mechanisms involve a complex interaction between the remaining dopamine system and the semi-homologous serotonin and norepinephrine systems. Serotonin and norepinephrine transporters (SERT and NET, respectively) have affinity for dopamine uptake especially when dopamine transporters (DAT) are scant. Monoamine reuptake inhibitors have been reported to modulate l-DOPA's anti-parkinsonian effects, but DAT, SERT, and NET's contribution to dyskinesia has not been well delineated. The current investigation sought to uncover the differential expression and function of DAT, SERT, and NET in the l-DOPA-treated hemi-parkinsonian rat. Protein analysis of striatal monoamine transporters in unilateral sham or 6-hydroxydopamine-lesioned rats treated with l-DOPA (0 or 6 mg/kg) showed lesion-induced DAT loss and l-DOPA-induced gain in SERT:DAT and NET:DAT ratios in lesioned rats which positively correlated with dyskinesia expression, suggesting functional shifts among monoamine transporters in the dyskinetic state. SERT blockade with citalopram (3, 5 mg/kg) reduced LID while DAT and NET blockade with GBR-12909 (5, 10 mg/kg) and nisoxetine (5, 10 mg/kg), respectively, mildly exacerbated dyskinesia expression. Transporter inhibition did not significantly alter l-DOPA's ability to reverse motor deficit. Overall, DA and DAT loss with l-DOPA treatment appear to precipitate gain in SERT and NET function. Strong correlations with LID and direct behavioral comparisons of selective transporter blockade reveal novel implications for SERT, DAT, and NET as potential biomarkers and therapeutic targets in the hemi-parkinsonian model and dyskinetic PD patients.

A working model for the assessment of disruptions in social behavior among aged rats: The role of sex differences, social recognition, and sensorimotor processes.

Aging results in a natural decline in social behavior, yet little is known about the processes underlying these changes. Engaging in positive social interaction is associated with many health benefits, including reduced stress reactivity, and may serve as a potential buffer against adverse consequences of aging. The goal of these studies was to establish a tractable model for the assessment of social behavior deficits associated with late aging. Thus, in Exp. 1, 1.5-, 3-, and 18-month-old male Fischer 344 (F344) rats were assessed for object investigation, and social interaction with a same-aged partner (novel/familiar), or a different-aged partner, thereby establishing working parameters for studies that followed. Results revealed that 18-month-old males exhibited reductions in social investigation and social contact behavior, with this age-related decline not influenced by familiarity or age of the social partner. Subsequently, Exp. 2 extended assessment of social behavior to both male and female F344 rats at multiple ages (3, 9, 18, and 24 months), after which a series of sensorimotor performance tests were conducted. In this study, both males and females exhibited late aging-related reductions in social interactions, but these changes were more pronounced in females. Additionally, sensorimotor performance was shown to be impaired in 24-month-olds, but not 18-month-olds, with this deficit more evident in males. Finally, Exp. 3 examined whether aging-related inflammation could account for declines in social behavior during late aging by administering naproxen (0, 7, 14, and 28 mg/kg; s.c.)-a non-steroidal anti-inflammatory drug-to 18-month-old females. Results from this study revealed that social behavior was unaffected by acute or repeated (6 days) naproxen, suggesting that aging-related social deficits in females may not be a consequence of a general aging-related inflammation and/or malaise. Together, these findings demonstrate that aging-related declines in social behavior are (i) specific to social stimuli and (ii) not indicative of a general state of aging-related debilitation. Thus, these findings establish working parameters for a highly tractable model in which the neural and hormonal mechanisms underlying aging-related declines in social behavior can be examined.

Effect of tricyclic antidepressants on L-DOPA-induced dyskinesia and motor improvement in hemi-parkinsonian rats.

Although dopamine replacement therapy with L-DOPA in Parkinson's disease initially reduces motor symptoms, its chronic use often leads to the development of abnormal involuntary movements known as L-DOPA-induced dyskinesia. Increasingly, research has indicated that non-dopaminergic neurons gain function in the parkinsonian brain, taking up and converting L-DOPA to dopamine and releasing it as a "false neurotransmitter". Although less explored, promiscuity between monoamine transporters may also modulate these processes. Therefore, in order to examine the differential roles of monoamine transporters in L-DOPA's behavioral effects, three tricyclic antidepressants (TCA) with graded affinity for serotonin (SERT) vs. norepinephrine (NET) transporters were tested in hemi-parkinsonian rats: clomipramine (SERT>NET), amitriptyline (SERT=NET), and desipramine (SERT

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.