Compensatory mechanisms are thought to maintain sufficient dopamine (DA) signaling to mitigate locomotor impairment during progressive nigrostriatal neuron loss in Parkinson’s disease (PD). Recent evidence indicated augmented DA tissue content in the substantia nigra (SN), not striatum, compensates for tyrosine hydroxylase (TH) and neuronal loss, and alleviates the severity of hypokinesia during neuronal loss. Here, we determined if increased extracellular DA in the SN may also be a compensatory mechanism to augment DA signaling. Following unilateral 6-hydroxydopamine (6-OHDA) lesion or sham-operation, we contemporaneously evaluated extracellular DA against both DA tissue and TH levels in striatum and SN at 7 and 28 days. At 7 days post-lesion, TH loss exceeded ~90% in striatum, and ~70% in the SN. The severity of DA tissue loss coincided with TH protein loss only in striatum (>90%) on both days after lesion, whereas in the SN, DA loss was absent on day 7 and significantly less than TH loss by day 28. Whereas there was a robust increase in extracellular DA in striatum in our sham-operation group, the severe TH and DA tissue loss in striatum practically abolished KCl (K+)-stimulated extracellular DA by day 7. In contrast, whereas striatal K+-stimulation had no effect on extracellular DA in the SN in sham-operation group, extracellular DA levels increased in the SN 7 days after nigrostriatal lesion: an increase no longer apparent by day 28. Thus, despite significant loss of TH protein loss in the SN, extracellular and tissue DA tissue levels were augmented during neuronal loss. These results build upon evidence that compensatory mechanisms to augment DA signaling are not engaged in striatum, and point to the SN as the locus of augmented DA signaling to offset loss of TH during nigrostriatal neuron loss.

Parkinson's Disease (PD) is a movement disorder characterized by the loss of nigrostriatal dopamine (DA) neurons. Early-stage PD motor symptoms are typically controlled with the DA replacement therapy, levodopa (L-DOPA). However, chronic use of this treatment causes abnormal involuntary movements (AIMs) called L-DOPA-induced dyskinesia (LID). Accumulating research suggests that exercise can modify the course of PD by modulating DA signaling, thereby improving movement, and thus may also slow LID progression. This study evaluated these phenomena in hemiparkinsonian rats that display severe unilateral DA loss. Sprague-Dawley rats received unilateral 6-hydroxydopamine (6-OHDA) lesions of the left medial forebrain bundle and were then counterbalanced into equally lesioned exercise or sedentary groups. Thereafter, rats underwent 3 weeks of treadmill exercise or exposure, respectively. One-hour post-treadmill, all animals received a 4 mg/kg subcutaneous (s.c.) injection of L-DOPA. All rats were rated weekly for LID using the AIMs test. Rotarod performance was conducted 24 h before or following AIMs. Following the 4-week exercise regimen, forepaw adjusting steps (FAS) and rotarod were conducted 60 min after L-DOPA to assess group differences in L-DOPA efficacy. Results revealed lesion-induced decrements in motor performance were significantly better within exercised subjects after L-DOPA treatment. Moreover, exercise reduced the severity of LID. Post-mortem analyses showed severe DA loss in the nigrostriatal pathway of both groups while 5-HIAA, 5-HT and DA turnover were significantly higher in the lesioned striatum (STR) of exercised animals. Thus, treadmill exercise optimized L-DOPA while reducing LID. Although exercise may not restore DA in cases of severe loss, enhanced functional compensation may support its benefits when paired with DA replacement therapy.

Parkinson’s disease (PD) is a progressive neurodegenerative disorder that is characterized by motor symptoms such as tremors, rigidity, and bradykinesia. Magnetic resonance imaging (MRI) offers a non-invasive means to study PD and its progression. This study utilized the unilateral 6-hydroxydopamine (6-OHDA) rat model of parkinsonism to assess whether white matter microstructural integrity measured using advanced free-water diffusion tensor imaging metrics (fw-DTI) and gray matter density using voxel-based morphometry (VBM) can serve as imaging biomarkers of pathological changes following nigrostriatal denervation. By comparing the 6-OHDA-lesioned vs. sham-lesioned rats, we aimed to identify complementary gray matter and white matter changes indicative of disease pathophysiology. Results showed widespread gray matter atrophy and subtle changes in white matter integrity in the 6-OHDA lesioned rats. Gray matter atrophy predominantly affected ipsilateral cortical regions, with some bilateral regions also showing atrophy. Conversely, higher volumes were observed in some regions of the contralateral gray matter in the 6-OHDA model. Furthermore, increased fw-FA and fw-AX were observed in regions including the brainstem, thalamus, superior and inferior colliculus, and fornix. Smaller clusters of decreased fw-FA and fw-AX were found in the corpus callosum. Regions of both increased and decreased diffusivity were noted in fw-RD, primarily in the brainstem, while the f index was elevated in several regions in the 6-OHDA lesioned group, except for a cluster in the contralateral thalamus. In conclusion, this study underscores the significant potential role for gray and white matter imaging biomarkers in delineating disease pathology in parkinsonism.

Standard treatment for Parkinson's disease (PD) is dopamine replacement therapy with L-DOPA. However, chronic treatment often results in abnormal involuntary movements called L-DOPA-induced dyskinesia (LID). Prior evidence indicates that heightened striatal cholinergic tone may contribute to LID. Restoring cholinergic inhibition by targeting the inhibitory M4 muscarinic acetylcholine (ACh) receptor (M4) reduces LID in preclinical models. Although intrinsic striatal sources of ACh have been considered for their role in LID, extrinsic sources of ACh such as the pedunculopontine nucleus (PPN) have not been well investigated for their role in LID. Therefore, the current study employed hemiparkinsonian Long-Evans rats with a PPN-targeted cannula ipsilateral to 6-OHDA lesion. We examined the effect of local unilateral PPN infusion of M4 PAM VU0467154 on LID, motor performance, and c-fos expression within the PPN. It was expected that PPN infusion of VU0467154 would reduce LID, reduce L-DOPA's motor benefit, and globally reduce c-fos expression in the PPN. Contrary to our expectations, PPN infusion of M4 PAM did not significantly affect LID severity. Furthermore, M4 PAM did not alter L-DOPA-mediated motor improvement, and decreased c-fos expression specifically in PPN cholinergic neurons. These results suggest that local PPN ACh dynamics differ from those of the striatum. In the context of prior work, our results suggest that PPN cholinergic modulation or global PPN modulation may be a promising strategy for altering freezing of gait without decreasing motor benefit of L-DOPA and without increasing LID severity.

[This corrects the article DOI: 10.3389/fncir.2024.1463941.].

IntroductionParkinson's Disease (PD), the second most common neurodegenerative disorder, is characterized by motor and non-motor symptoms linked to dopaminergic neuron degeneration. This study utilized the 6-hydroxydopamine (6-OHDA) rat model to replicate PD-like dopaminergic degeneration through targeted injections into the medial forebrain bundle and substantia nigra.MethodsBehavioral assessments revealed hallmark motor deficits, while MRI was performed to assess complementary functional connectivity and structural connectivity. Post-mortem tyrosine hydroxylase (TH) staining confirmed extensive dopaminergic neuron loss, validating the pathological relevance of the model and ensuring data integrity. MRI data were collected at 7T in 46 male Fischer F344 rats (23 6-OHDA, 23 sham) to characterize functional and structural connectivity differences between cohorts.ResultsFunctionally, decreased connectivity between the retrosplenial and endopiriform cortices in the 6-OHDA model suggests disrupted sensory processing, while increased connectivity between the hippocampus and retrosplenial cortex indicates possible compensatory mechanisms. Structurally, we observed reduced connectivity between the subcoeruleum and piriform cortex in the 6-OHDA model, which may reflect axonal degeneration, and increased connectivity between the ventral striatum and primary somatosensory cortex, which likely reflects compensatory changes to support motor-sensory integration. Diffusion MRI analysis further revealed changes in the white matter tracts connecting these regions, supporting these findings and highlighting adaptive responses to neurodegeneration in PD.DiscussionThese findings demonstrate the utility of combining functional and structural connectivity analyses to capture PD-related network disruptions. These structural connectivity changes were further associated with microstructural alterations. The development of MRI biomarkers for understanding brain connectivity may enhance our understanding of PD pathology and advancing translation of these techniques to clinical applications.

During progressive nigrostriatal neuron loss in Parkinson’s disease (PD), compensatory mechanisms are thought to maintain dopamine (DA) signaling at levels sufficient to mitigate locomotor impairment prior to substantial neuron loss. Whereas increased DA turnover in striatum has been considered a keystone compensatory mechanism indicative of augmented DA signaling, recent work indicates that increased DA biosynthesis in substantia nigra (SN), not striatum, compensates for tyrosine hydroxylase (TH) protein and neuronal loss. To extend interrogation of compensatory mechanisms that augment DA signaling during nigrostriatal neuron loss, we contemporaneously evaluated extracellular DA against tissue DA levels and quantified TH protein in the striatum and SN. Our unilateral 6-hydroxydopamine (6-OHDA) lesion approach produces progressive neuronal loss between 7 and 28 days, enabling evaluation of multiple DA signaling components in striatum vs SN. Loss of TH was ∼90% in striatum and ∼70% in the SN by 7 days after lesion induction. However, whereas loss of tissue DA matched TH loss in striatum (>90%) on both days after lesion, tissue DA loss in SN occurred only at day 28 (36%), despite major TH loss by day 7. This preservation of nigral DA tissue levels during lesion progression was associated with increased extracellular DA in SN after K+-dependent depolarization in striatum, which was not evident in a sham-operation group at the same time, early after lesion induction, signifying a DA lesion-specific adaptation in SN. In contrast in the striatum, lesion abolished the robust increase in extracellular DA, as seen in the sham-operation group. Together, these results indicate compensatory mechanisms that augment nigrostriatal DA signaling are engaged in the SN, not striatum, during progressive nigrostriatal neuron loss. ### Competing Interest Statement The authors have declared no competing interest. US Department of Defense U.S. Army Medical Research and Material Command CDMRP

Obesity's metabolic impact goes beyond peripheral insulin resistance, influencing the brain and thereby disrupting energy homeostasis by altering anabolic and catabolic reactions. Density-enhanced phosphatase-1 (DEP-1), a ubiquitously expressed receptor-like tyrosine phosphatase, has emerged as a novel regulator with a specific role in dephosphorylating the insulin receptor (IR). Strikingly, diet-induced obese mice exhibit elevated DEP-1 expression in metabolically sensitive tissues. In this study, we sought to elucidate the role of DEP-1 in brain insulin signaling and highlight its potential impact on the central regulation of metabolism in vitro and in vivo. To explore DEP-1 deficiency in vitro, CRISPR/Cas9 was employed to create a DEP-1 knockout (KO) in mouse Neuro-2a cells. Given DEP-1’s high expression in the forebrain and the region's abundant IR expression, we examined the metabolic consequences of DEP-1 deficiency in the forebrain by deleting Dep-1 using CamKIIa Cre mice (DEFO KO). DEP-1 KO cells exhibited an increase in IR phosphorylation and downstream signaling upon acute insulin stimulation, coupled with unexpected activation of AMP-activated protein kinase (AMPK) cascade. Similarly, male DEFO KO mice showed heightened insulin and AMPK signaling in the forebrain under random-fed conditions. A notable outcome manifested in the gonadal white adipose tissue of DEFO KO mice, where an ex vivo lipolysis assay revealed elevated basal lipolytic potential compared to control mice stimulated with isoproterenol, indicating enhanced sympathetic activation. Brown adipose tissue (BAT) of DEFO KO mice also demonstrated in a mitochondrial respiration assay, an upregulation of fatty acid oxidation, signifying increased BAT activity. Both tissues also revealed higher beta-adrenergic receptor gene expression, suggestive of sympathetic activity. In conclusion, DEP-1 is a pivotal neuronal IR phosphatase, that prevents the establishment of futile cycles and emerges as a novel modulator of energy metabolism. Disclosure S. Chopra: None. O.L.J. Kadiri: None. J. Ulke: None. R. Hauffe: None. W. Jonas: None. C.A. Bishop: None. S. Cheshmeh: None. M. Rath: None. M. Schell: None. K. Kappert: None. A. Kleinridders: Speaker's Bureau; Novo Nordisk A/S, Daiichi Sankyo. Funding German Research Foundation (DFG)KL2399/6-1

Parkinson’s disease (PD) is a neurodegenerative disorder typified by the loss of dopamine (DA) neurons in the substantia nigra pars compacta (SNpc) leading to motor symptoms including resting tremor, rigidity, akinesia, and postural instability. DA replacement therapy with levodopa (L-DOPA) remains the gold-standard treatment for the motor symptoms of PD. Unfortunately, chronic use of L-DOPA leads to the development of side effects known as L-DOPA-induced dyskinesia (LID). The mechanisms underlying LID are multifaceted, but accumulating research has strongly implicated maladaptive neuroplasticity within the raphe-striatal serotonin (5-HT) circuit. The 5-HT transporter (SERT) has emerged as an intriguing therapeutic target as it is upregulated in the brains of dyskinetic patients and animal models of LID, and pharmacological blockade of SERT alters L-DOPA’s effects. Therefore, the current study employed an interventional genetic knockdown of SERT (SERT-KD) to investigate its role in LID expression and LID-associated transcription factors. To do so, hemiparkinsonian, stably dyskinetic rats (N=68) received adeno-associated virus 9 (AAV9) expressing either a short-hairpin RNA against SERT (SERT-shRNA) or a scrambled control shRNA (SCR-shRNA) after which LID reinstatement and motor performance were assayed over 2 weeks. Dorsal raphe and striatal tissue were collected for the expression analyses of known parkinsonian and LID-associated genes. Results demonstrated that SERT-KD significantly and durably reduced LID and L-DOPA-induced striatal cFOS mRNA without altering L-DOPA efficacy. Such findings point to SERT-mediated adaptations as a 5-HT mechanism by which L-DOPA exerts its actions and therapeutic target for LID.

Alleviation of motor impairment by aerobic exercise (AE) in Parkinson's disease (PD) patients points to activation of neurobiological mechanisms that may be targetable by therapeutic approaches. However, evidence for AE-related recovery of striatal dopamine (DA) signaling or tyrosine hydroxylase (TH) loss has been inconsistent in rodent studies. This ambiguity may be related to the timing of AE intervention in relation to the status of nigrostriatal neuron loss. Here, we replicated human PD at diagnosis by establishing motor impairment with >80% striatal DA and TH loss prior to initiating AE, and assessed its potential to alleviate motor decline and restore DA and TH loss. We also evaluated if serum levels of neurofilament light (NfL) and glial fibrillary acidic protein (GFAP), biomarkers of human PD severity, changed in response to AE. 6-hydroxydopamine (6-OHDA) was infused unilaterally into rat medial forebrain bundle to induce progressive nigrostriatal neuron loss over 28 days. Moderate intensity AE (3x per week, 40 min/session), began 8-10 days post-lesion following establishment of impaired forelimb use. Striatal tissue DA, TH protein and mRNA, and serum levels of NfL/GFAP were determined 3-wks after AE began. Despite severe striatal DA depletion at AE initiation, forelimb use deficits and hypokinesia onset were alleviated by AE, without recovery of striatal DA or TH protein loss, but reduced NfL and GFAP serum levels. This proof-of-concept study shows AE alleviates motor impairment when initiated with >80% striatal DA loss without obligate recovery of striatal DA or TH protein. Moreover, the AE-related reduction of NfL and GFAP serum levels may serve as objective blood-based biomarkers of AE efficacy.

Parkinson's Disease (PD) is a neurodegenerative movement disorder characterized by dopamine (DA) cell loss in the substantia nigra pars compacta (SNc). As PD progresses, patients display disruptions in gait such as changes in posture, bradykinesia, and shortened stride. DA replacement via L-DOPA alleviates many PD symptoms, though its effects on gait are not well demonstrated. This study aimed to assess the relationship between DA lesion, gait, and deficit-induced reversal with L-DOPA. To do so, Sprague-Dawley rats (N = 25, 14 males, 11 females) received unilateral medial forebrain bundle (MFB) DA lesions with 6-hydroxydopamine (6-OHDA). An automated gait analysis system assessed spatiotemporal gait parameters pre- and post-lesion, and after various doses of L-DOPA (0, 3, or 6 mg/kg; s.c.). The forepaw adjusting steps (FAS) test was implemented to evaluate lesion efficacy while the abnormal involuntary movements (AIMs) scale monitored the emergence of L-DOPA-induced dyskinesia (LID). High performance liquid chromatography (HPLC) assessed changes in brain monoamines on account of lesion and treatment. Results revealed lesion-induced impairments in gait, inclusive of max-contact area and step-sequence alterations that were not reversible with L-DOPA. However, the emergence of AIMs were observed at higher doses. Post-mortem, 6-OHDA lesions induced a loss of striatal DA and norepinephrine (NE), while prefrontal cortex (PFC) displayed noticeable reduction in NE but not DA. Our findings indicate that hemiparkinsonian rats display measurable gait disturbances similar to PD patients that are not rescued by DA replacement. Furthermore, non-DA mechanisms such as attention-related NE in PFC may contribute to altered gait and may constitute a novel target for its treatment.

IntroductionParkinson’s disease (PD) is commonly characterized by severe dopamine (DA) depletion within the substantia nigra (SN) leading to a myriad of motor and non-motor symptoms. One underappreciated and prevalent non-motor symptom, Parkinson’s disease-associated psychosis (PDAP), significantly erodes patient and caregiver quality of life yet remains vastly understudied. While the gold standard pharmacotherapy for motor symptoms Levodopa (LD) is initially highly effective, it can lead to motor fluctuations like LD-induced dyskinesia (LID) and non-motor fluctuations such as intermittent PDAP. One source of these fluctuations could be the serotonergic raphe nuclei and their projections. Serotonin (5-HT) neurons possess the machinery necessary to convert and release DA from exogenous LD. In DA-depleted brain regions these 5-HT projections can act as surrogates to the DA system initially compensating but chronically leading to aberrant neuroplasticity which has been linked to LID and may also contribute to non-motor fluctuations. In support, recent work from our lab established a positive relationship between LID and PDAP in parkinsonian rats. Therefore, it was hypothesized that normalizing 5-HT forebrain input would reduce the co-expression of LID and PDAP.MethodsTo do so, we expressed 5-HT projection specific inhibitory designer receptor exclusively activated by designer drugs (DREADDs) using Cre-dependent AAV9-hM4di in tryptophan hydroxylase 2 (TPH2)-Cre bilaterally 6-OHDA-lesioned rats. Thereafter we used the designer drug Compound 21 to selectively inhibit 5-HT raphe projections during LD treatment to modulate the expression of PDAP, assayed by prepulse inhibition (PPI) and LID, quantified by the abnormal involuntary movements (AIMs) test.ResultsOur results suggest that chemogenetic inhibition of 5-HT raphe-projecting cells significantly reduces LID without affecting stepping ability or established sensorimotor gating deficitsDiscussionOverall, this study provides further evidence for the complex influence of 5-HT raphe-projecting neurons on LD’s neurobehavioral effects.

Parkinson's disease (PD) is characterized by the accumulation of misfolded alpha-synuclein (α-syn) protein, forming intraneuronal Lewy body (LB) inclusions. The α-syn preformed fibril (PFF) model of PD recapitulates α-syn aggregation, progressive nigrostriatal degeneration and motor dysfunction; however, little is known about the time course of PFF-induced alterations in basal and evoked dopamine (DA). In vivo microdialysis is well suited for identifying small changes in neurotransmitter levels over extended periods. In the present study, adult male Fischer 344 rats received unilateral, intrastriatal injections of either α-syn PFFs or phosphate-buffered saline (PBS). At 4 or 8 months post-injection (p.i.), animals underwent in vivo microdialysis to evaluate basal extracellular striatal DA and metabolite levels, local KCl-evoked striatal DA release and the effects of systemic levodopa ( l -DOPA). Post-mortem analysis demonstrated equivalent PFF-induced reductions in tyrosine hydroxylase (TH) immunoreactive nigral neurons (~50%) and striatal TH (~20%) at both time points. Compared with reduction in striatal TH, reduction in striatal dopamine transporter (DAT) was more pronounced and progressed between the 4- and 8-month p.i. intervals (36% ➔ 46%). Significant PFF-induced deficits in basal and evoked striatal DA, as well as deficits in motor performance, were not observed until 8 months p.i. Responses to l -DOPA did not differ regardless of PBS or PFF treatment. These results suggest that basal and evoked striatal DA are maintained for several months following PFF injection, with loss of both associated with motor dysfunction. Our studies provide insight into the time course and magnitude of PFF-induced extracellular dopaminergic deficits in the striatum.

Standard treatment for Parkinson’s disease (PD) is dopamine replacement therapy with L-DOPA. However, chronic treatment often results in abnormal involuntary movements called L-DOPA-induced dyskinesia (LID). Prior evidence indicates that heightened striatal cholinergic tone may contribute to LID. Restoring cholinergic inhibition by targeting the inhibitory M4 muscarinic acetylcholine (ACh) receptor (M4) reduces LID in preclinical models. Although intrinsic striatal sources of ACh have been considered for their role in LID, extrinsic sources of ACh such as the pedunculopontine nucleus (PPN) have not been well investigated for their role in LID. Therefore, the current study employed hemiparkinsonian Long-Evans rats with a PPN-targeted cannula ipsilateral to 6-OHDA lesion. Following chronic treatment with L-DOPA, we examined the effect of local unilateral PPN infusion of M4 PAM VU0467154 on LID, motor performance, and c-fos expression within the PPN. It was expected that PPN infusion of VU0467154 would reduce LID, reduce L-DOPA’s motor benefit, and globally reduce c-fos expression in the PPN. Contrary to our expectations, PPN infusion of M4 PAM did not significantly affect LID severity. Furthermore, the group receiving M4 PAM showed slightly elevated motor improvement compared to L-DOPA, and decreased c-fos expression specifically in PPN cholinergic neurons. These results suggest that local PPN ACh dynamics differ from those of the striatum. Specifically, our results suggest that PPN cholinergic neurons may be a promising therapeutic target for augmenting L-DOPA-mediated motor benefit without increasing LID. ### Competing Interest Statement The authors have declared no competing interest. * 6-OHDA : 6-hydroxydopamine ACh : Acetylcholine AIMs : Abnormal involuntary movements scale ALO : Axial, limb, and orolingual ChAT : Choline acetyltransferase DA : Dopamine D1 : Dopamine D1 receptor FAS : Forepaw adjusting steps GABA : Gamma-aminobutyric acid IHC : Immunohistochemistry IR : Immunoreactivity LID : L-DOPA-induced dyskinesia M4 : M4 muscarinic acetylcholine receptor PBS : Phosphate buffered saline PD : Parkinson’s disease PPN : Pedunculopontine nucleus rPPN : Rostral pedunculopontine nucleus TH : Tyrosine hydroxylase

Background Alleviation of motor impairment by aerobic exercise (AE) in Parkinson’s disease (PD) points to a CNS response that could be targeted by therapeutic approaches, but recovery of striatal dopamine (DA) or tyrosine hydroxylase (TH) has been inconsistent in rodent studies. Objective To increase translation of AE, 3 components were implemented into AE design to determine if recovery of established motor impairment, concomitant with >80% striatal DA and TH loss, was possible. We also evaluated if serum levels of neurofilament light (NfL) and glial fibrillary acidic protein (GFAP), blood-based biomarkers of disease severity in human PD, were affected. Methods We used a 6-OHDA hemiparkinson rat model featuring progressive nigrostriatal neuron loss over 28 days, with impaired forelimb use 7 days post-lesion, and hypokinesia onset 21 days post-lesion. After establishing forelimb use deficits, moderate intensity AE began 1-3 days later, 3x per week, for 40 min/session. Motor assessments were conducted weekly for 3 wks, followed by determination of striatal DA, TH protein and mRNA, and NfL and GFAP serum levels. Results Seven days after 6-OHDA lesion, recovery of depolarization-stimulated extracellular DA and DA tissue content was <10%, representing severity of DA loss in human PD, concomitant with 50% reduction in forelimb use. Despite severe DA loss, recovery of forelimb use deficits and alleviation of hypokinesia progression began after 2 weeks of AE and was maintained. Increased NfLand GFAP levels from lesion were reduced by AE. Despite these AE-driven changes, striatal DA tissue and TH protein levels were unaffected. Conclusions This proof-of-concept study shows AE, using exercise parameters within the capabilities most PD patients, promotes recovery of established motor deficits in a rodent PD model, concomitant with reduced levels of blood-based biomarkers associated with PD severity, without commensurate increase in striatal DA or TH protein.

L-DOPA is the standard treatment for Parkinson’s disease (PD), but chronic treatment typically leads to L-DOPA-induced dyskinesia (LID). LID involves a complex interaction between the remaining dopamine (DA) system and the semi-homologous serotonin (5-HT) system. Since serotonin transporters (SERT) have some affinity for DA uptake, they may serve as a functional compensatory mechanism when DA transporters (DAT) are scant. DAT and SERT’s functional contributions in the dyskinetic brain have not been well delineated. The current investigation sought to determine how DA depletion and L-DOPA treatment affect DAT and SERT transcriptional processes, translational processes, and functional DA uptake in the 6-hydroxydopamine-lesioned hemi-parkinsonian rat. Rats were counterbalanced for motor impairment into equally lesioned treatment groups then given daily L-DOPA (0 or 6 mg/kg) for 2 weeks. At the end of treatment, the substantia nigra was processed for tyrosine hydroxylase (TH) and DAT gene expression and dorsal raphe was processed for SERT gene expression. The striatum was processed for synaptosomal DAT and SERT protein expression and ex vivo DA uptake. Nigrostriatal DA loss severely reduced DAT mRNA and protein expression in the striatum with minimal changes in SERT. L-DOPA treatment, while not significantly affecting DAT or SERT alone, did increase striatal SERT:DAT protein ratios. Using ex vivo microdialysis, L-DOPA treatment increased DA uptake via SERT when DAT was depleted. Overall, these results suggest that DA loss and L-DOPA treatment uniquely alter DAT and SERT, revealing implications for monoamine transporters as potential biomarkers and therapeutic targets in the hemi-parkinsonian model and dyskinetic PD patients.