From unilateral to bilateral parkinsonism: Effects of lateralization on dyskinesias and associated molecular mechanisms

Lack of correlation between dyskinesia and pallidal serotonin transporter expression-induced by L-Dopa and Pramipexole in hemiparkinsonian rats

The role of pallidal serotonergic terminals in the development of L-Dopa-induced dyskinesias (LIDs) in Parkinson's disease (PD) has been recently highlighted correlating pallidal serotonin transporter (SERT) expression levels with dyskinesias severity. However, the role of external globus pallidus (GPe, GP in rodents) serotonergic function in LIDs is still controversial since several studies have shown no differences in GPe serotonin (SER) and SERT levels between dyskinetic and non-dyskinetic PD patients. In addition, the increase in pallidal SERT/dopamine transporter (DAT) binding ratio obtained in positron emission tomography studies has been shown similar in both subtypes of PD patients. Based on these controversial results, further studies are required to clarify the possible involvement of GPe serotonergic activity in LIDs expression. We investigated the pallidal SER and SERT expression changes and the abnormal involuntary movements (AIMs) induced by L-Dopa or the D3/D2 dopamine (DA) agonist, Pramipexole, in partial unilateral 6-hydroxydopamine (6-OHDA)-lesioned rats. L-Dopa treatment led to an increment of axial (p < 0.01), limb (p < 0.01), and orolingual (p < 0.01) AIMs. However, Pramipexole treatment did not induce AIMs. The number of GP SERT-positive axon varicosities was increased in L-Dopa (p < 0.05) and Pramipexole (p < 0.01) treated rats. No differences were observed in the number of GP SERT-positive varicosities between L-Dopa and Pramipexole treatments. Our results indicate a lack of correlation between GP SERT expression levels and the development of AIMs suggesting that pallidal serotonergic fibers are not responsible for LIDs. The possible involvement of the SER system in dyskinesia may include other mechanisms.

The Rodent Models of Dyskinesia and Their Behavioral Assessment

Dyskinesia, a major motor complication resulting from dopamine replacement treatment, manifests as involuntary hyperkinetic or dystonic movements. This condition poses a challenge to the treatment of Parkinson's disease. So far, several behavioral models based on rodent with dyskinesia have been established. These models have provided an important platform for evaluating the curative effect of drugs at the preclinical research level over the past two decades. However, there are differences in the modeling and behavioral testing procedures among various laboratories that adversely affect the rat and mouse models as credible experimental tools in this field. This article systematically reviews the history, the pros and cons, and the controversies surrounding rodent models of dyskinesia as well as their behavioral assessment protocols. A summary of factors that influence the behavioral assessment in the rodent dyskinesia models is also presented, including the degree of dopamine denervation, stereotaxic lesion sites, drug regimen, monitoring styles, priming effect, and individual and strain differences. Besides, recent breakthroughs like the genetic mouse models and the bilateral intoxication models for dyskinesia are also discussed.

Gastrointestinal non-motor dysfunction in Parkinson's disease model rats with 6-hydroxydopamine

Parkinson's disease (PD) is a neurodegenerative disease with a progressive loss of mesencephalic dopaminergic neurons of the substantia nigra (SN). To further evaluate its pathophysiology, accurate animal models are needed. The current study aims to verify the impact of a 6-hydroxydopamine (6-OHDA) bilateral microinjection into the SN on gastrointestinal symptoms in rats and confirm that the 6-OHDA rat model is an appropriate tool to investigate the mechanisms of Parkinsonian GI disorders. Immunohistochemistry, digital X-ray imaging, short-circuit current, FITC-dextran permeability and ultra-performance liquid chromatography tandem mass spectrometry were used in this study. The results indicated that the dopaminergic neurons in SN and fibres in the striatum were markedly reduced in 6-OHDA rats. The 6-OHDA rats manifested reductions in occupancy in a rotarod test and increases in daily food debris but no difference in body mass or daily consumption. Compared with control rats, faecal pellets and their contents were significantly decreased, whereas gastric emptying and intestinal transport were delayed in 6-OHDA rats. The increased in vivo FITC-dextran permeability and decreased intestinal transepithelial resistance in the model suggest attenuated barrier function in the digestive tract in the PD model. Moreover, inflammatory factors in the plasma showed that pro-inflammatory factors IL-1? and IL-8 were significantly increased in 6-OHDA rats. Collectively, these findings indicate that the model is an interesting experimental tool to investigate the mechanisms involved in the progression of gastrointestinal dysfunction in PD.

Levodopa/Benserazide Loaded Microspheres Alleviate L-dopa Induced Dyskinesia through Preventing the Over-Expression of D1R/Shp-2/ERK1/2 Signaling Pathway in a Rat Model of Parkinson's Disease

Background: The long-term intermittent Levodopa (L-dopa) stimulation contributes to an aberrant activation of D1 receptor (D1R) mediated extracellular signal-regulated kinases1/2 (ERK1/2) in the striatal medium spiny neurons, resulting in the occurrence of L-dopa induced dyskinesia (LID). Recently, a novel signaling pathway, D1R/Shp-2/ERK1/2, was proposed to be required for the occurrence of LID. Here we designed the study in which two different methods of L-dopa delivery [continuous dopamine stimulation (CDS) vs. intermittent dopamine stimulation] were used to further identify: (1) the role of D1R/Shp-2/ERK1/2 signaling pathway in the occurrence of LID; (2) whether CDS alleviated LID though preventing the over-expression of the D1R/Shp-2/ERK1/2 signaling pathway. Methods: 6-OHDA-lesioned rat models of Parkinson's disease (PD) were randomly divided into two groups to receive intermittent L-dopa stimulation (L-dopa/benserazide standard group, LS group) or CDS (L-dopa/benserazide loaded microspheres, LBM group) for 21 days. Dyskinesia and anti-parkinsonian effect were compared between the two groups through the AIMs assessment and cylinder test. The critical protein changes in the D1R/Shp-2/ERK1/2 signaling pathway were compared between the two groups through Western blotting. Results: Intermittent L-dopa administration induced serious dyskinetic movements in the 6-OHDA-lesioned rats, and the anti-parkinsonian effect of L-dopa was gradually counteracted by the occurrence of dyskinesia. Intermittent L-dopa administration enhanced the expression of membrane D1R, and induced a robust increase of phosphorylation of Shp-2, Src, DARPP-32, and ERK1/2 in the 6-OHDA-lesioned striatum. In contrast, CDS played a dose-dependent anti-parkinsonian role, without inducing such apparent dyskinetic movements. Moreover, CDS induced no change of membrane D1R expression or phosphorylation of Shp-2, Src, DARPP-32, and ERK1/2 in the 6-OHDA-lesioned striatum. Conclusion: The aberrant activation of D1R/Shp-2 complex was evidenced to be required for the D1R mediating ERK1/2 phosphorylation and the occurrence of LID. CDS effectively prevented the overexpression of D1R/Shp-2/ERK1/2 signaling pathway, resulting in the reduction of LID in 6-OHDA-lesioned rats model of PD.

Antidyskinetic Treatment with MTEP Affects Multiple Molecular Pathways in the Parkinsonian Striatum

Parkinson's disease is characterized by dopaminergic neuron loss and dopamine (DA) depletion in the striatum. Standard treatment is still focused on the restoration of dopamine with exogenous L-Dopa, which however causes L-Dopa-induced dyskinesia (LID). Several studies have shown that antagonism of the metabotropic glutamate receptor 5 alleviates LID, but the underlying mechanisms have remained unclear. We set out to determine where this alleviation may depend on restoring the equilibrium between the two main striatofugal pathways. For this purpose, we examined molecular markers of direct and indirect pathway involvement (prodynorphin and proenkephalin, resp.) in a rat model of LID treated with the mGluR5 antagonist MTEP. Our results show that MTEP cotreatment significantly attenuates the upregulation of prodynorphin mRNA induced by L-Dopa while also decreasing the expression levels of proenkephalin mRNA. We also examined markers of the mGluR5-related PKC/MEK/ERK1/2 signaling pathway, finding that both the expression of PKC epsilon and the phosphorylation of MEK and ERK1/2 had decreased significantly in the MTEP-treated group. Taken together, our results show that pharmacological antagonism of mGluR5 normalizes several abnormal molecular responses in the striatum in this experimental model of LID.

Olfactory function in an excitotoxic model for secondary neuronal degeneration: Role of dopaminergic interneurons

Secondary neuronal degeneration (SND) occurring in Traumatic brain injury (TBI) consists in downstream destructive events affecting cells that were not or only marginally affected by the initial wound, further increasing the effects of the primary injury. Glutamate excitotoxicity is hypothesized to play an important role in SND. TBI is a common cause of olfactory dysfunction that may be spontaneous and partially recovered. The role of the glutamate excitotoxicity in the TBI-induced olfactory dysfunction is still unknown. We investigated the effects of excitotoxicity induced by bilateral N-Methyl-D-Aspartate (NMDA) OB administration in the olfactory function, OB volumes, and subventricular zone (SVZ) and OB neurogenesis in rats. NMDA OB administration induced a decrease in the number of correct choices in the olfactory discrimination tests one week after lesions (p<0.01), and a spontaneous recovery of the olfactory deficit two weeks after lesions (p<0.05). A lack of correlation between OB volumes and olfactory function was observed. An increase in SVZ neurogenesis (Ki67+ cells, PSANCAM+ cells (p<0.01) associated with an increase in OB glomerular dopaminergic immunostaining (p<0.05) were related to olfactory function recovery. The present results show that changes in OB volumes cannot explain the recovery of the olfactory function and suggest a relevant role for dopaminergic OB interneurons in the pathophysiology of recovery of loss of smell in TBI.

Short- and long-term effects induced by repeated 6-OHDA intraventricular administration: A new progressive and bilateral rodent model of Parkinson's disease

The pathological hallmark of Parkinson's disease (PD) is the progressive degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNpc), and the resulting striatal dopamine deficiency, which are responsible for the classic motor features. Although a diagnosis of PD relies on the clinical effects of dopamine deficiency, this disease is also associated with other neurotransmitter deficits that are recognized as causing various motor and non-motor symptoms. However, the cause of dopaminergic nigral neurodegeneration in PD and the underlying mechanisms remain unknown. While animal models are considered valuable tools with which to investigate dopaminergic cell vulnerability, rodent models usually fail to mimic the neurodegeneration progression that occurs in human PD. To find a convenient rat model for studying the progression of dopaminergic cell degeneration and motor signs, we have developed a progressive rodent model using a repeated daily, intraventricular administration of the neurotoxin 6-hydroxydopamine (6-OHDA) (100µg/day) in awakened rats for 1 to 10 consecutive days. The short- (6-day) and long-term (32-day) progression of motor alterations was studied. This model leads to a bilateral and progressive increase in catalepsy (evident from the 3rd infusion in the short-term groups (p<0.01) and from the 7th infusion in the long-term groups (p<0.01), which was associated with a progressive nigrostriatal dopaminergic deficit. All together this makes the new model an interesting experimental tool to investigate the mechanisms involved in the progression of dopaminergic neurodegeneration.

Neural substrates and potential treatments for levodopa-induced dyskinesias in Parkinson's disease

Parkinson's disease (PD) is primarily a motor disorder that involves the gradual loss of motor function. Symptoms are observed initially in the extremities, such as hands and arms, while advanced stages of the disease can effect blinking, swallowing, speaking, and breathing. PD is a neurodegenerative disease, with dopaminergic neuronal loss occurring in the substantia nigra pars compacta, thus disrupting basal ganglia functions. This leads to downstream effects on other neurotransmitter systems such as glutamate, γ-aminobutyric acid, and serotonin. To date, one of the main treatments for PD is levodopa. While it is generally very effective, prolonged treatments lead to levodopa-induced dyskinesia (LID). LID encompasses a family of symptoms ranging from uncontrolled repetitive movements to sustained muscle contractions. In many cases, the symptoms of LID can cause more grief than PD itself. The purpose of this review is to discuss the possible clinical features, cognitive correlates, neural substrates, as well as potential psychopharmacological and surgical (including nondopaminergic and deep brain stimulation) treatments of LID.

Ibuprofen or piroxicam protects nigral neurons and delays the development of l-dopa induced dyskinesia in rats with experimental Parkinsonism: Influence on angiogenesis

Neuroinflammation and angiogenesis have been involved in the pathogenesis of Parkinson's disease (PD). This study investigated the effect of ibuprofen or piroxicam on the motor response to l-dopa and development of dyskinesia in Parkinsonian rats focusing on the anti-angiogenic role of the two non-steroidal anti-inflammatory drugs (NSAIDs). Rats were divided into nine groups as follows: Group I: the vehicle group, Group II: rotenone group, rats were injected with nine doses of rotenone (1 mg/kg/48 h), group III&IV: rats received rotenone + ibuprofen (10 or 30 mg/kg), Group V-VI: rats received rotenone + piroxicam (1 or 3 mg/kg), Group VII: rats received rotenone + l-dopa/carbidopa (100/10 mg/kg), Group VIII-IX: rats received rotenone + l-dopa/carbidopa + ibuprofen (30 mg/kg) or piroxicam (3 mg/kg). In general, drugs were administered daily for ten weeks. Rotenone-treated rats showed motor dysfunction, lower striatal dopamine, lower staining for nigral tyrosine hydroxylase but higher level of striatal cyclooxygenase-2 (COX-2) and vascular endothelial growth factor (VEGF) compared to vehicle-treated rats (P < 0.05). Treatment with l-dopa showed wearing-off over the course of the experiment in addition to development of abnormal involuntary movements and upregulated striatal VEGF level. Treatment with ibuprofen or piroxicam in combination with l-dopa preserved the effect of l-dopa at the end of week 10, delayed the development of dyskinesia and decreased striatal COX-2 and VEGF levels. In conclusion, the current study suggests that ibuprofen and piroxicam are promising candidates for neuroprotection in PD and may have utility in conjunction with l-dopa in order to ensure the longevity of its action and to delay the development of dyskinesia.