Regulation of cortical and striatal 5-HT1A receptors in the MPTP-lesioned macaque

Striatal serotonin transporter gain-of-function in L-DOPA-treated, hemi-parkinsonian rats

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.

The effects of Vilazodone, YL-0919 and Vortioxetine in hemiparkinsonian rats

Parkinson's disease is a neurodegenerative disease often characterized by motor deficits and most commonly treated with dopamine replacement therapy. Despite its benefits, chronic use of L-DOPA results in abnormal involuntary movements known as L-DOPA-induced dyskinesia. Growing evidence shows that with burgeoning dopamine cell loss, neuroplasticity in the serotonin system leads to the development of L-DOPA-induced dyskinesia through the unregulated uptake, conversion, and release of L-DOPA-derived dopamine into the striatum. Previous studies have shown that coincident 5-HT_1A agonism and serotonin transporter inhibition may have anti-dyskinetic potential. Despite this, few studies have explicitly focused on targeting both 5-HT_1A and the serotonin transporter. The present study compares the 5-HT compounds Vilazodone, YL-0919, and Vortioxetine which purportedly work as simultaneous 5-HT_1A receptor agonists and SERT blockers. To do so, adult female Sprague Dawley rats were rendered hemiparkinsonian and treated daily for two weeks with L-DOPA to produce stable dyskinesia. The abnormal involuntary movements and forehand adjusting step tests were utilized as measurements for L-DOPA-induced dyskinesia and motor performance in a within-subjects design. Lesion efficacy was determined by analysis of striatal monoamines via high-performance liquid chromatography. Compounds selective for 5-HT_1A/SERT target sites including Vilazodone and Vortioxetine significantly reduced L-DOPA-induced dyskinesia without compromising L-DOPA pro-motor efficacy. In contrast, YL-0919 failed to reduce L-DOPA-induced dyskinesia, with no effects on L-DOPA-related improvements. Collectively, this work supports pharmacological targeting of 5-HT_1A/SERT to reduce L-DOPA-induced dyskinesia. Additionally, this further provides evidence for Vilazodone and Vortioxetine, FDA-approved compounds, as potential adjunct therapeutics for L-DOPA-induced dyskinesia management in Parkinson's patients.

Different Alterations of Agonist and Antagonist Binding to 5-HT1A Receptor in a Rat Model of Parkinson’s Disease and Levodopa-Induced Dyskinesia: A MicroPET Study

Background: The gold-standard treatment for Parkinson’s disease is L-DOPA, which in the long term often leads to levodopa-induced dyskinesia. Serotonergic neurons are partially responsible for this, by converting L-DOPA into dopamine leading to its uncontrolled release as a “false neurotransmitter”. The stimulation of 5-HT1A receptors can reduce involuntary movements but this mechanism is poorly understood. Objective: This study aimed to investigate the functionality of 5-HT1A receptors using positron emission tomography in hemiparkinsonian rats with or without dyskinesia induced by 3-weeks daily treatment with L-DOPA. Imaging sessions were performed “off” L-DOPA. Methods: Each rat underwent a positron emission tomography scan with [18F]F13640, a 5-HT1AR agonist which labels receptors in a high affinity state for agonists, or with [18F]MPPF, a 5-HT1AR antagonist which labels all the receptors. Results: There were decreases of [18F]MPPF binding in hemiparkinsonian rats in cortical areas. In dyskinetic animals, changes were slighter but also found in other regions. In hemiparkinsonian rats, [18F]F13640 uptake was decreased bilaterally in the globus pallidus and thalamus. On the non-lesioned side, binding was increased in the insula, the hippocampus and the amygdala. In dyskinetic animals, [18F]F13640 binding was strongly increased in cortical and limbic areas, especially in the non-lesioned side. Conclusion: These data suggest that agonist and antagonist 5-HT1A receptor-binding sites are differently modified in Parkinson’s disease and levodopa-induced dyskinesia. In particular, these observations suggest a substantial involvement of the functional state of 5-HT1AR in levodopa-induced dyskinesia and emphasize the need to characterize this state using agonist radiotracers in physiological and pathological conditions.

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.

The role of neurotransmitters in the development of Parkinson's disease-related psychosis

Psychotic symptoms are common, disabling non-motor features of Parkinson's disease (PD). Despite noted heterogeneity in clinical features, natural history and therapy response, current dogma posits that psychosis generally progresses in a stereotypic manner through a cascade of events that begins with minor hallucinations and evolves to severe hallucinations and delusions. Further, the occurrence of psychotic symptoms is believed to indicate a poor prognosis. Here we propose a classification scheme that outlines the pathogenesis of psychosis as it relates to dysfunction of several neurotransmitter systems. We hypothesize that several subtypes exist, and that PD psychosis is not consistently indicative of a progressive cascade and poor prognosis. The literature was reviewed from 1990 to 2017. An overview of the features of PD psychosis is followed by a review of data indicating the existence of neurotransmitter-related subtypes of psychosis. We found that ample evidence exists to demonstrate the presence of multiple subtypes of PD psychosis, which are traced to dysfunction of the following neurotransmitter systems: dopamine, serotonin and acetylcholine. Dysfunction of each of these systems is recognizable through their clinical features and correlates, and the varied long-term prognoses. Identifying which neurotransmitter system is dysfunctional may help to develop targeted therapies. PD psychosis has various subtypes that differ in clinical features, underlying pathology and pathophysiology, treatment response and prognosis. A novel classification scheme is presented that describes the clinical subtypes with different outcomes, which could lead to the development of targeted therapies. Future research should focus on testing the viability of this classification.

Serotonergic Approaches in Parkinson's Disease: Translational Perspectives, an Update

Parkinson's disease (PD) has long been seen as a disorder caused by degeneration of the dopaminergic system, leading to the classic motor manifestations of the disease. However, there is now overwhelming evidence that PD is more than a disease merely caused by dopamine depletion. It is well-known that a myriad of other neurotransmitters are affected by the disease process. One such neurotransmitter is serotonin (5-HT). 5-HT has been shown to play a role in several motor and nonmotor manifestations of PD, including tremor, cognition, depression and psychosis. 5-HT also seems to play a critical role in L-3,4-dihydroxyphenylalanine (L-DOPA)-induced dyskinesia. A breadth of preclinical studies and clinical trials have been conducted that aimed at modulating the 5-HT system in order to alleviate depression, cognitive deficits, psychosis, and dyskinesia. In this Review, we summarize recent advances in the 5-HT field in PD, but with a translational emphasis. We start by presenting a novel nonhuman primate model of PD that presents with dual dopamine and 5-HT lesions. We then present preclinical and clinical data that introduce new concepts, such as the use of biased and partial agonists, as well as molecules recently introduced to the field of PD, such as eltoprazine, pimavanserin, nelotanserin, and SYN-120, to enhance therapeutic benefit while minimizing adverse events, notably on parkinsonian disability.

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.

Alteration in Nuclear Factor-KappaB Pathway and Functionality of Estrogen via Receptors Promote Neuroinflammation in Frontal Cortex after 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine Treatment

The MPTP mediated neurodegeneration in substantia nigra has been well studied, but not the status of frontal cortex. The novelty of the present study is to explore the sex difference of frontal cortex during MPTP intoxication and to investigate the role of estrogen and its receptors in presence of glial cells in a time chase experiment; to identify which pathway of NF-kappaB exist to proceed the neuroinflammation; to investigate the estrogen binding with its nuclear or cytosolic receptors and whether any direct relation exists between estrogen receptor (ER) -beta and NF-kappaB molecules p65 and RelB. The progression of neurodegeneration occurred with the association of glial cells and functional (via its nuclear and cytosolic receptors) estrogen level. Both the canonical and/or non canonical pathways of NF-kappaB exist in frontal cortex of both the sexes after MPTP treatment. The homodimeric or heterodimeric form of ER-beta binds with NF-kappaB molecules p65 and RelB differently, but the canonical or non canonical pathways of NF-kappaB molecules could not be stopped or may be promoted. The changes in the molecular and cellular pattern in frontal cortex of both sexes during MPTP intoxication depends on the estrogen function via its nuclear or cytosolic estrogen receptors.

Dyskinesias and impulse control disorders in Parkinson's disease: From pathogenesis to potential therapeutic approaches

Dopaminergic treatment in Parkinson's disease (PD) reduces the severity of motor symptoms of the disease. However, its chronic use is associated with disabling motor and behavioral side effects, among which levodopa-induced dyskinesias (LID) and impulse control disorders (ICD) are the most common. The underlying mechanisms and pathological substrate of these dopaminergic complications are not fully understood. Recently, the refinement of imaging techniques and the study of the genetics and molecular bases of LID and ICD indicate that, although different, they could share some features. In addition, animal models of parkinsonism with LID have provided important knowledge about mechanisms underlying such complications. In contrast, animal models of parkinsonism and abnormal impulsivity, although useful regarding some aspects of human ICD, do not fully resemble the clinical phenotype of ICD in patients with PD, and until now have provided limited information. Studies on animal models of addiction could complement the previous models and provide some insights into the background of these behavioral complications given that ICD are regarded as behavioral addictions. Here we review the most relevant advances in relation to imaging, genetics, biochemistry and pharmacological interventions to treat LID and ICD in patients with PD and in animal models with a view to better understand the overlapping and unique maladaptations to dopaminergic therapy that are associated with LID and ICD.

The highly-selective 5-HT(1A) agonist F15599 reduces L-DOPA-induced dyskinesia without compromising anti-parkinsonian benefits in the MPTP-lesioned macaque

L-3,4-dihydroxyphenylalanine (L-DOPA) is the most effective anti-parkinsonian agent available, but upon chronic administration, patients with Parkinson's disease (PD) experience abnormal involuntary movements, dyskinesia. Modulation of serotonin 1A (5-HT1A) receptors is regarded as an effective way to alleviate dyskinesia, yet this approach has been marred by a reduction of the therapeutic effectiveness of L-DOPA. We hypothesised that highly-selective 5-HT1A stimulation might be a way to alleviate dyskinesia without compromising L-DOPA anti-parkinsonian action. F15599 (also known as NLX-101) is a highly-selective 5-HT1A agonist that displays over 1000 × selectivity over off-target receptors. Seven cynomolgus macaques were administered MPTP and developed severe parkinsonism. Following chronic administration of L-DOPA, they developed severe and reproducible dyskinesia. F15599 (0.003, 0.01, 0.03 and 0.1 mg/kg) or vehicle was administered in combination with L-DOPA and its effect on dyskinesia and L-DOPA anti-parkinsonian was assessed. In combination with L-DOPA, F15599 (0.1 mg/kg) reduced the severity of peak-dose dyskinesia, by ≈45% (P < 0.001), compared to L-DOPA alone. F15599 (any dose) had no effect on duration of on-time or motor activity counts compared to L-DOPA alone. F15599 at 0.03 and 0.1 mg/kg significantly reduced duration of on-time with disabling dyskinesia (by ≈49% and ≈71%, P < 0.05 and P < 0.001, respectively). These results suggest that F15599, a highly-selective 5-HT1A receptor agonist, alleviates dyskinesia without exerting a deleterious effect on L-DOPA anti-parkinsonian action.

Serotonin impairment in CSF of PD patients, without an apparent clinical counterpart

In Parkinson's disease (PD), several studies have detected an impaired serotonin (5-HT) pathway, likely affecting both motor and non-motor domains. However, the precise impact of 5-HT impairment is far from established. Here, we have used a HPLC chromatographic method, in a homogenous cohort (n = 35) of non fluctuating, non dyskinetic PD patients, to assess the concentration of 5-HT and its metabolite 5-HIAA in peripheral cerebrospinal fluid (CSF) obtained from lumbar puncture (LP). LP was performed following three days of therapy withdrawal, in order to vanish the effects of prolonged released dopamine agonists (DA), and in absence of any serotonergic agent. The PD patient group showed a significantly reduced CSF level of both 5-HT and 5-HIAA compared to either age-matched control subjects (n = 18), or Alzheimer's disease patients (n = 20). However, no correlation emerged between 5-HT/5-HIAA concentrations and UPDRS-III (r = −0.12), disease duration (r = −0.1), age (r = −0.27) and MMSE (r = 0.11). Intriguingly, low CSF 5-HT levels did not differ for gender or for motor phenotype (in terms of non-tremor dominant subtype and tremor dominant subtype). Further, low CSF 5-HT levels did not correlate with the presence of depression, apathy or sleep disturbance. Our findings support the contention that 5-HT impairment is a cardinal feature of stable PD, probably representing a hallmark of diffuse Lewy bodies deposition in the brainstem. However, clinical relevance remains uncertain. Given these findings, an add-on therapy with serotonergic agents seems questionable in PD patients, or should be individually tailored, unless severe depression is present.

Interaction between the 5-HT system and the basal ganglia: functional implication and therapeutic perspective in Parkinson's disease

The neurotransmitter serotonin (5-HT) has a multifaceted function in the modulation of information processing through the activation of multiple receptor families, including G-protein-coupled receptor subtypes (5-HT1, 5-HT2, 5-HT4-7) and ligand-gated ion channels (5-HT3). The largest population of serotonergic neurons is located in the midbrain, specifically in the raphe nuclei. Although the medial and dorsal raphe nucleus (DRN) share common projecting areas, in the basal ganglia (BG) nuclei serotonergic innervations come mainly from the DRN. The BG are a highly organized network of subcortical nuclei composed of the striatum (caudate and putamen), subthalamic nucleus (STN), internal and external globus pallidus (or entopeduncular nucleus in rodents, GPi/EP and GPe) and substantia nigra (pars compacta, SNc, and pars reticulata, SNr). The BG are part of the cortico-BG-thalamic circuits, which play a role in many functions like motor control, emotion, and cognition and are critically involved in diseases such as Parkinson's disease (PD). This review provides an overview of serotonergic modulation of the BG at the functional level and a discussion of how this interaction may be relevant to treating PD and the motor complications induced by chronic treatment with L-DOPA.

5-HT(1A) receptor: an old target as a new attractive tool in drug discovery from central nervous system to cancer

The serotonin receptor subtype 5-HT(1A) was one of the first serotonin receptor subtypes pharmacologically characterized. This receptor subtype has long been object of intense research and is implicated in the pathogenesis and treatment of anxiety and depressive disorders. In recent years, new chemical entities targeting the 5-HT(1A) receptor (alone or in combination with other molecular targets) have been proposed for novel therapeutic uses in neuroprotection, cognitive impairment, Parkinson's disease, pain treatment, malignant carcinoid syndrome, and prostate cancer. This Perspective compares existing data on expression and signaling activity of the 5-HT(1A) receptor to a ligand with an intrinsic agonist or antagonist profile. Our purpose is also to make a complete overview, useful for underlining the features needed to select a specific pharmacological profile rather than another one. This aspect could be really interesting to consider and justify the 5-HT(1A) receptor as a new attractive target for drug discovery.

Effects of 5-HT1A receptor stimulation on striatal and cortical M1 pERK induction by L-DOPA and a D1 receptor agonist in a rat model of Parkinson's disease

Motor symptoms of Parkinson's disease are commonly treated using l-DOPA although long-term treatment usually causes debilitating motor side effects including dyskinesias. A putative source of dyskinesia is abnormally high levels of phosphorylated extracellular-regulated kinase (pERK) within the striatum. In animal models, the serotonin 1A receptor agonist ±8-OH-DPAT reduces dyskinesia, suggesting it may exhibit efficacy through the pERK pathway. The present study investigated the effects of ±8-OH-DPAT on pERK density in rats treated with l-DOPA or the D1 receptor agonist SKF81297. Rats were given a unilateral dopamine lesion with 6-hydroxydopamine and primed with a chronic regimen of l-DOPA, SKF81297 or their vehicles. On the final test day, rats were given two injections: first with ±8-OH-DPAT, the D1 receptor antagonist SCH23390 or their vehicles, and second with l-DOPA, SKF81297 or their vehicles. Rats were then transcardially perfused for immunohistological analysis of pERK expression in the striatum and primary motor cortex. Rats showed greater dyskinesia in response to l-DOPA and SKF81297 after repeated injections. Although striatal pERK induction was similar between acute and chronic l-DOPA, SKF81297 caused the largest increase in striatal pERK after the first exposure. Neither compound alone affected motor cortex pERK. Surprisingly, in the ventromedial striatum, ±8-OH-DPAT potentiated l-DOPA-induced pERK; in the motor cortex, ±8-OH-DPAT potentiated pERK with l-DOPA or SKF81297. Our results support previous work that the striatal pERK pathway is dysregulated after dopamine depletion, but call into question the utility of pERK as a biomarker of dyskinesia expression.

Effects of 5-HT1A receptor stimulation on D1 receptor agonist-induced striatonigral activity and dyskinesia in hemiparkinsonian rats

Accumulating evidence supports the value of 5-HT1A receptor (5-HT1AR) agonists for dyskinesias that arise with long-term L-DOPA therapy in Parkinson's disease (PD). Yet, how 5-HT1AR stimulation directly influences the dyskinetogenic D1 receptor (D1R)-expressing striatonigral pathway remains largely unknown. To directly examine this, one cohort of hemiparkinsonian rats received systemic injections of Vehicle + Vehicle, Vehicle + the D1R agonist SKF81297 (0.8 mg/kg), or the 5-HT1AR agonist ±8-OH-DPAT (1.0 mg/kg) + SKF81297. Rats were examined for changes in abnormal involuntary movements (AIMs), rotations, striatal preprodynorphin (PPD), and glutamic acid decarboxylase (GAD; 65 and 67) mRNA via RT-PCR. In the second experiment, hemiparkinsonian rats received intrastriatal pretreatments of Vehicle (aCSF), ±8-OH-DPAT (7.5 mM), or ±8-OH-DPAT + the 5-HT1AR antagonist WAY100635 (4.6 mM), followed by systemic Vehicle or SKF81297 after which AIMs, rotations, and extracellular striatal glutamate and nigral GABA efflux were measured by in vivo microdialysis. Results revealed D1R agonist-induced AIMs were reduced by systemic and intrastriatal 5-HT1AR stimulation while rotations were enhanced. Although ±8-OH-DPAT did not modify D1R agonist-induced increases in striatal PPD mRNA, the D1R/5-HT1AR agonist combination enhanced GAD65 and GAD67 mRNA. When applied locally, ±8-OH-DPAT alone diminished striatal glutamate levels while the agonist combination increased nigral GABA efflux. Thus, presynaptic 5-HT1AR stimulation may attenuate striatal glutamate levels, resulting in diminished D1R-mediated dyskinetic behaviors, but maintain or enhance striatal postsynaptic factors ultimately increasing nigral GABA levels and rotational activity. The current findings offer a novel mechanistic explanation for previous results concerning 5-HT1AR agonists for the treatment of dyskinesia.

Increased levels of 5-HT1A receptor binding in ventral visual pathways in Parkinson's disease

Visual hallucinations are common in advanced Parkinson's disease (PD). The pathophysiology of visual hallucinations may involve enhanced serotonergic neurotransmission. The atypical antipsychotics clozapine and quetiapine, which have affinity for 5-HT(2A) and 5-HT(1A) receptors, are effective against visual hallucinations in PD. 5-HT(2A) receptors are increased in ventral visual pathways in PD patients with visual hallucinations, and we hypothesized that 5-HT(1A) receptors were also involved in visual hallucinations in PD. Autoradiographic binding using [(3) H]-WAY-100,635 and NAN-190 was performed in brain sections from 6 PD patients with visual hallucinations, 6 PD patients without visual hallucinations, and 5 age-matched controls. All PD subjects had been treated with L-dopa. Brain areas studied were the orbitofrontal, inferolateral temporal, and motor cortices, as well as the striatum, globus pallidus, substantia nigra, and thalamus. 5-HT(1A) -binding levels were dramatically increased in the ventral visual pathways of all PD patients compared with controls (0 vs 11 and 0 vs 100 nmol/mg, respectively; both P < .05). There was no significant difference in 5-HT(1A) -binding levels in PD patients with visual hallucinations compared with PD patients without visual hallucinations or with controls in any of the brain areas studied (P > .05). Gross abnormalities in 5-HT(1A) levels in ventral visual areas occurred in all PD patients exposed to L-dopa. However, as there was no difference in 5-HT(1A) -binding levels between hallucinators and nonhallucinators, alterations in 5-HT(1A) receptor levels may not contribute specifically to visual hallucinations in PD. However, the discrete anatomical distribution of rises to the ventral visual areas suggests some role in predisposing to visual hallucinations.

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.

The serotonergic system in Parkinson's disease

Although the cardinal manifestations of Parkinson's disease (PD) are attributed to a decline in dopamine levels in the striatum, a breadth of non-motor features and treatment-related complications in which the serotonergic system plays a pivotal role are increasingly recognised. Serotonin (5-HT)-mediated neurotransmission is altered in PD and the roles of the different 5-HT receptor subtypes in disease manifestations have been investigated. The aims of this article are to summarise and discuss all published preclinical and clinical studies that have investigated the serotonergic system in PD and related animal models, in order to recapitulate the state of the current knowledge and to identify areas that need further research and understanding.

Vascular endothelial growth factor is upregulated by L-dopa in the parkinsonian brain: implications for the development of dyskinesia

Angiogenesis and increased permeability of the blood-brain barrier have been reported to occur in animal models of Parkinson's disease and l-dopa-induced dyskinesia, but the significance of these phenomena has remained unclear. Using a validated rat model of l-dopa-induced dyskinesia, this study demonstrates that chronic treatment with l-dopa dose dependently induces the expression of vascular endothelial growth factor in the basal ganglia nuclei. Vascular endothelial growth factor was abundantly expressed in astrocytes and astrocytic processes in the proximity of blood vessels. When co-administered with l-dopa, a small molecule inhibitor of vascular endothelial growth factor signalling significantly attenuated the development of dyskinesia and completely blocked the angiogenic response and associated increase in blood-brain barrier permeability induced by the treatment. The occurrence of angiogenesis and vascular endothelial growth factor upregulation was verified in post-mortem basal ganglia tissue from patients with Parkinson's disease with a history of dyskinesia, who exhibited increased microvascular density, microvascular nestin expression and an upregulation of vascular endothelial growth factor messenger ribonucleic acid. These congruent findings in the rat model and human patients indicate that vascular endothelial growth factor is implicated in the pathophysiology of l-dopa-induced dyskinesia and emphasize an involvement of the microvascular compartment in the adverse effects of l-dopa pharmacotherapy in Parkinson's disease.

Role of the primary motor cortex in L-Dopa-induced dyskinesia and its modulation by 5-HT1A receptor stimulation

While serotonin 5-HT1A receptor (5-HT1AR) agonists reduce L-DOPA-induced dyskinesias (LID) by normalizing activity in the basal ganglia neurocircuitry, recent evidence suggests putative 5-HT1AR within the primary motor cortex (M1) may also contribute. To better characterize this possible mechanism, c-fos immunohistochemistry was first used to determine the effects of systemic administration of the full 5-HT1AR agonist ±8-OH-DPAT on L-Dopa-induced immediate early gene expression within M1 and the prefrontal cortex (PFC) of rats with unilateral medial forebrain bundle (MFB) dopamine (DA) lesions. Next, in order to determine if direct stimulation of 5-HT1AR within M1 attenuates the onset of LID, rats with MFB lesions were tested for L-Dopa-induced abnormal involuntary movements (AIMs) and rotations following M1 microinfusions of ±8-OH-DPAT with or without coadministration of the 5-HT1AR antagonist WAY100635. Finally, ±8-OH-DPAT was infused into M1 at peak dyskinesia to determine if 5-HT1AR stimulation attenuates established L-Dopa-induced AIMs and rotations. While no treatment effects were seen within the PFC, systemic ±8-OH-DPAT suppressed L-Dopa-induced c-fos within M1. Intra-M1 5-HT1AR stimulation diminished the onset of AIMs and this effect was reversed by WAY100635 indicating receptor specific effects. Finally, continuous infusion of ±8-OH-DPAT into M1 at peak dyskinesia alleviated L-Dopa-induced AIMs. Collectively, these findings support an integral role for M1 in LID and its modulation by local 5-HT1AR.

Local modulation of striatal glutamate efflux by serotonin 1A receptor stimulation in dyskinetic, hemiparkinsonian rats

Serotonin 1A receptor (5-HT(1A)R) agonists reduce both L-DOPA- and D1 receptor (D1R) agonist-mediated dyskinesia, but their anti-dyskinetic mechanism of action is not fully understood. Given that 5-HT(1A)R stimulation reduces glutamatergic neurotransmission in the dopamine-depleted striatum, 5-HT(1A)R agonists may diminish dyskinesia in part through modulation of pro-dyskinetic striatal glutamate levels. To test this, rats with unilateral medial forebrain bundle dopamine or sham lesions were primed with L-DOPA (12 mg/kg+benserazide, 15 mg/kg, sc) or the D1R agonist SKF81297 (0.8 mg/kg, sc) until abnormal involuntary movements (AIMs) stabilized. On subsequent test days, rats were treated with vehicle or the 5-HT(1A)R agonist ±8-OH-DPAT (1.0 mg/kg, sc), followed by L-DOPA or SKF81297, or intrastriatal ±8-OH-DPAT (7.5 or 15 mM), followed by L-DOPA. In some cases, the 5-HT(1A)R antagonist WAY100635 was employed to determine receptor-specific effects. In vivo microdialysis was used to collect striatal samples for analysis of extracellular glutamate levels during AIMs assessment. Systemic and striatal ±8-OH-DPAT attenuated L-DOPA-induced dyskinesia and striatal glutamate efflux while WAY100635 reversed ±8-OH-DPAT's effects. Interestingly, systemic ±8-OH-DPAT diminished D1R-mediated AIMs without affecting glutamate. These findings indicate a novel anti-dyskinetic mechanism of action for 5-HT(1A)R agonists with implications for the improved treatment of Parkinson's disease.