Impaired cognitive and motor function are coincident with L-DOPA-induced dyskinesia in a model of Parkinson's disease.

Dopamine transmission has been implicated in motor and cognitive function. In Parkinson's disease (PD), dopamine replacement using the precursor drug L-DOPA is the predominant treatment approach, but long-term exposure leads to the onset of dyskinesias (LIDs). Chronic L-DOPA exposure has been associated with changes in gene expression and altered cortico-striatal plasticity. The aim of this research was to assess the functional consequence of long-term L-DOPA exposure on cognitive and motor function using a rodent model of PD. Across two independent experiments, we assessed the impact of chronic L-DOPA exposure, or a control D2R agonist, on motor and cognitive function in intact and in hemi parkinsonian rats, in the absence of drug. Abnormal involuntary movements associated with LID were measured and brain tissues were subsequently harvested for immunohistochemical analysis. Exposure to chronic L-DOPA, but not the D2R agonist, impaired motor and cognitive function, when animals were tested in the absence of drug. A meta-analysis of the two experiments allowed further dissociation of L-DOPA -treated rats into those that developed LIDs (dyskinetic) and those that did not develop LIDs (non-dyskinetic). This analysis revealed impaired cognitive and motor performance were evident only in dyskinetic, but not in non-dyskinetic, rats. These data reveal a functional consequence of the altered plasticity associated with LID onset and have implications for understanding symptom progression in the clinic.

Rodent models of impulsive-compulsive behaviors in Parkinson's disease: How far have we reached?

There is increasing awareness that the medications used to treat the motor symptoms of Parkinson's disease (PD) contribute to the development of behavioral addictions, which have been clinically defined as impulsive-compulsive behaviors (ICBs). These features include pathological gambling, compulsive sexual behavior, binge eating, compulsive shopping, excessive hobbyism or punding, and the excessive use of dopaminergic medication. ICBs frequently have devastating effects on the social and occupational function of the affected individuals as well as their families. Although ICBs are an important clinical problem in PD, the number of studies in which these symptoms have been modeled in rodents is still limited. This may depend on uncertainties regarding, on one hand, the pathophysiology of these behaviors and, on the other hand, the experimental paradigms with which similar features can be induced in rodents. To help compose these uncertainties, we will here review the characteristics of ICBs in PD patients and then describe behavioral methods to approximate them in rodents. We will discuss both the challenges and the possibilities of applying these methods to animals with PD-like lesions, and review the recent progress made to this end. We will finally highlight important questions deserving further investigation. Rodent models having both face validity and construct validity to parkinsonian ICBs will be essential to further pathophysiological and therapeutic studies into this important area.

Nigral 6-hydroxydopamine lesion impairs performance in a lateralised choice reaction time task--impact of training and task parameters.

Unilateral intrastriatal and intra-medial forebrain bundle injections of 6-OHDA impair the performance in a lateralised choice reaction time task. However, the extent and pattern of deficits after nigral 6-OHDA injections is less well studied, as well as the impact of training regime or the modification of various task parameters. The nigral 6-OHDA lesion resulted in impaired response accuracy and an increased time to react to and execute the response on the side contralateral to the lesion as compared to sham-lesioned controls. Pre-training of the rats on the task prior to the lesion resulted in slightly faster reaction times as well as a reduced number of preservative panel presses compared to when rats were trained after the 6-OHDA injection. When the rat had to perform a longer sustained nose poke before responding to the lateralised stimuli, the number of useable trials was reduced in both controls and 6-OHDA rats as a result of an increased number of premature withdrawals from the centre hole. This study demonstrates that rats with a nigral 6-OHDA lesion display several distinct deficits in this operant task, which are similar to those seen after striatal and bundle 6-OHDA injections. In addition, by combining pre-training with the use of a short set of holds, improved sensitivity of this task can be achieved. This improvement in sensitivity may be of advantage when exploring new therapeutic interventions for PD, where subtle but relevant changes in performance may arise.

The effect of additional noradrenergic and serotonergic depletion on a lateralised choice reaction time task in rats with nigral 6-OHDA lesions.

Parkinson's disease (PD) patients often suffer from visuospatial deficits, which have been considered a disruption of the representation of external space. The lateralised choice reaction time (CRT) task is an operant task for rodents in which similar deficits can be assessed. It has been demonstrated that specific parameters in this task is disrupted after unilateral injections of 6-hydroxydopamine (6-OHDA), which have been associated with the dopamine (DA) depletion that inevitably follows this type of lesion. However, studies have demonstrated that this type of lesion also affects the serotonergic (5HT) and noradrenergic (NA) systems. However, the impact of these systems on parameters in the CRT task had not yet been investigated. To this end, rats were pretrained on the CRT task before receiving selective lesions of the DAergic system, either alone or in combination with depletion of the NA or 5HT system. All rats with a 6-OHDA lesion displayed a gradual decline in the selection, initiation and execution of lateralised movements compared to sham-lesion controls on the side contralateral to the lesion. They also displayed a reduced number of useable trials as well as an increased number of procedural errors. Interestingly, the group with an additional noradrenergic lesion was significantly slower in reacting to lateralised stimuli throughout the testing period compared to the other two groups with a 6-OHDA lesion. There was however no difference between the three different lesion groups in the other parameters assessed in the task. These data confirm previous findings demonstrating that the majority of the parameters assessed in the lateralised CRT task are strongly dependent on DA. However, this study has also shown that the NAergic system may play an important role in contributing to the attentive performance influencing the capacity to react to the presented lateralised stimuli.

Lesions of the dorsomedial striatum impair formation of attentional set in rats.

Behavioural flexibility refers to the ability to rapidly adapt to novel situations and it has been suggested that the frontal lobe and basal ganglia are implicated in various components of adjusting to changes in environmental contingencies. Behavioural flexibility can be assessed using attentional set-shifting tasks, in which performance is impaired after damage to the prefrontal cortex. The present study explores the downstream contribution of the prefrontal projection zone in the dorsomedial striatum (DMS) to attentional set shifting. Rats were tested in two set-shifting tasks following quinolinic acid injections bilaterally into the DMS. When tested in a rodent version of the set-shifting task, rats with a DMS lesion displayed a greater number of errors during the reversal stages of the task than sham lesion controls but the nature of the errors did not differ between the two groups. Interestingly, when the rats were tested in a modified version of the set-shifting task, directly designed for measuring the formation of an attentional set, sham lesion controls displayed a pronounced shift-cost, evident of successful set-formation. In contrast, rats with DMS lesions failed to form an attentional set, showing no performance cost when a shift of attention was required. These results support previous reports of the importance of the DMS in behavioural flexibility but also suggest that this region is vital for the formation of set, possibly by extrapolating different perceptions into a unified representation of a dimension.

Do α-synuclein vector injections provide a better model of Parkinson's disease than the classic 6-hydroxydopamine model?

Improvements in modelling Parkinson's disease in rodents contribute to the advancement of scientific knowledge and open innumerable pathways for the development of new therapeutic interventions. In a recent article in this journal, Decressac and co-workers present an interesting comparison between two classic 6-hydroxydopamine (6-OHDA) models and the more recently established rodent model of Parkinson's disease induced by over-expression of α-synuclein using adeno-associated viral vectors. As expected, injections of 6-OHDA result in extensive loss of dopamine associated with pronounced motor deficits. Interestingly, over-expression of α-synuclein in the substantia nigra pars compacta also results in a considerable loss of dopamine as well as motor impairments. Both the level of dopamine loss and the motor deficits seen after α-synuclein over-expression were similar in extent to that seen after intrastriatal injections of 6-OHDA, but the temporal profile of degeneration and the development of motor deficits were progressive, more closely mimicking the clinical condition. This commentary offers further insights into the differences between these two rodent models, and asks how well they each replicate idiopathic PD. In addition, the translational relevance, reliability, and predictive value of this more recently developed AAV α-synuclein model are considered.

Cognitive dysfunction and depression in Parkinson's disease: what can be learned from rodent models?

Parkinson's disease (PD) has for decades been considered a pure motor disorder and its cardinal motor symptoms have been attributed to the loss of dopaminergic (DAergic) neurons in the substantia nigra pars compacta and to nigral Lewy body pathology. However, there has more recently been a shift in the conceptualization of the disease, and its pathological features have now been recognized as involving several other areas of the brain and indeed even outside the central nervous system. There are a corresponding variety of intrinsic non-motor symptoms such as autonomic dysfunction, cognitive impairment, sleep disturbances and neuropsychiatric problems, which cannot be explained exclusively by nigral pathology. In this review, we will focus on cognitive impairment and affective symptoms in PD, and we will consider whether, and how, these deficits can best be modelled in rodent models of the disorder. As only a few of the non-motor symptoms respond to standard DA replacement therapies, the quest for a broader therapeutic approach remains a major research effort, and success in this area in particular will be strongly dependent on appropriate rodent models. In addition, better understanding of the different models, as well as the advantages and disadvantages of the available behavioural tasks, will result in better tools for evaluating new treatment strategies for PD patients suffering from these neuropsychological symptoms.

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.

L-DOPA-induced dopamine efflux in the striatum and the substantia nigra in a rat model of Parkinson's disease: temporal and quantitative relationship to the expression of dyskinesia.

L-DOPA-induced dyskinesia in Parkinson's disease is associated with large increases in brain dopamine (DA) levels following drug dosing, but the precise significance of this phenomenon is not understood. Here we compare DA efflux and metabolism in the striatum and the substantia nigra in dyskinetic and non-dyskinetic animals following a standard dose of L-DOPA. Rats with 6-hydroxydopamine lesions were treated chronically with L-DOPA, monitored on the abnormal involuntary movements scale, and then subjected to intracerebral microdialysis under freely-moving conditions. Following s.c. L-DOPA injection, peak extracellular DA levels in both striatum and substantia nigra were about twice as large in dyskinetic animals compared to non-dyskinetic rats. This effect was not attributable to differences in DOPA levels or DA metabolism. The larger DA efflux in dyskinetic animals was blunted by 5-HT1A/5-HT1B receptor agonists and tetrodotoxin infusion, reflecting release from serotonin neurons. Striatal levels of serotonin and its main metabolite, 5-hydroxyindolacetic acid were indeed elevated in dyskinetic animals compared to non-dyskinetic rats, indicating a larger serotonergic innervation density in the former group. High DA release was, however, not sufficient to explain dyskinesia. The 'abnormal involuntary movements output' per unit concentration of striatal extracellular DA was indeed much larger in dyskinetic animals compared to non-dyskinetic cases at most time points examined. The present results indicate that both a high DA release post-L-DOPA administration and an increased responsiveness to DA must coexist for a full expression of dyskinesia.

Differential involvement of D1 and D2 dopamine receptors in L-DOPA-induced angiogenic activity in a rat model of Parkinson's disease.

Angiogenesis occurs in the brains of Parkinson's disease patients, but the effects of dopamine replacement therapy on this process have not been examined. Using rats with 6-hydroxydopamine lesions, we have compared angiogenic responses induced in the basal ganglia by chronic treatment with either L-DOPA, or bromocriptine, or a selective D1 receptor agonist (SKF38393). Moreover, we have asked whether L-DOPA-induced angiogenesis can be blocked by co-treatment with either a D1- or a D2 receptor antagonist (SCH23390 and eticlopride, respectively), or by an inhibitor of extracellular signal-regulated kinases 1 and 2 (ERK1/2) (SL327). L-DOPA, but not bromocriptine, induced dyskinesia, which was associated with endothelial proliferation, upregulation of immature endothelial markers (nestin) and downregulation of endothelial barrier antigen in the striatum and its output structures. At a dose inducing dyskinesia (1.5 mg/kg/day), SKF38393 elicited angiogenic changes similar to L-DOPA. Antagonism of D1- but not D2 class receptors completely suppressed both the development of dyskinesia and the upregulation of angiogenesis markers. In fact, L-DOPA-induced endothelial proliferation was markedly exacerbated by low-dose D2 antagonism (0.01 mg/kg eticlopride). Inhibition of ERK1/2 by SL327 attenuated L-DOPA-induced dyskinesia and completely inhibited all markers of angiogenesis. These results highlight the specific link between treatment-induced dyskinesias and microvascular remodeling in the dopamine-denervated brain. L-DOPA-induced angiogenesis requires stimulation of D1 receptors and activation of ERK1/2, whereas the stimulation of D2 receptors seems to oppose this response.

l-DOPA dosage is critically involved in dyskinesia via loss of synaptic depotentiation.

The emergence of levodopa (l-DOPA)-induced dyskinesia and motor fluctuations represents a major clinical problem in Parkinson's disease (PD). While it has been suggested that the daily dose of l-DOPA can play a critical role, the mechanisms linking l-DOPA dosage to the occurrence of motor complications have not yet been explored. Using an experimental model of PD we have recently demonstrated that long-term l-DOPA treatment leading to the induction of abnormal involuntary movements (AIMs) alters corticostriatal bidirectional synaptic plasticity. Dyskinetic animals, in fact, lack the ability to reverse previously induced long-term potentiation (LTP). This lack of depotentiation has been associated to a defect in erasing unessential motor information. Here chronic l-DOPA treatment was administered at two different doses to hemiparkinsonian rats, and electrophysiological recordings were subsequently performed from striatal spiny neurons. Both low and high doses of l-DOPA restored normal LTP, which was disrupted following dopamine (DA) denervation. By the end of the chronic treatment, however, while the low l-DOPA dose induced AIMs only in half of the rats, the high dose caused motor complications in all the treated animals. Interestingly, the dose-related expression of motor complications was associated with a lack of synaptic depotentiation. Our study provides further experimental evidence to support a direct correlation between the daily dosage of l-DOPA and the induction of motor complications and establishes a critical pathophysiological link between the lack of synaptic depotentiation and the expression of AIMs.

The "motor complication syndrome" in rats with 6-OHDA lesions treated chronically with L-DOPA: relation to dose and route of administration.

L-DOPA-induced motor complications can be modelled in rats with 6-hydroxydopamine (6-OHDA) lesions by chronic injections of L-DOPA. We have compared the sensitisation and duration of rotational responses, and the occurrence of dose-failure episodes and abnormal involuntary movements (AIMs) in 6-OHDA-lesioned rats with regard to the dose and route of administration of L-DOPA. Rats were treated with either low (6mg/kg) or high (25mg/kg) doses of L-DOPA twice daily for 21 days whereas control animals received injections of either saline or bromocriptine (2.5mg/kg). A dose-dependent and gradual development of AIMs and contralateral turning was observed in rats treated chronically with l-DOPA. Rats treated with bromocriptine exhibited rotational sensitisation but no AIMs. A shortening of motor response duration was not seen in any of the drug-treated groups. In contrast, dose-failure episodes occurred frequently in both L-DOPA- and bromocriptine-treated animals. Changing the route of L-DOPA administration from intraperitoneal to subcutaneous completely abolished failures in motor response without affecting the development of dyskinesia. Based on the hypothesis that higher doses of L-DOPA may be toxic to dopaminoceptive structures, we compared the total number of neurons and the levels of activated microglia in the striatum. No signs of neurodegenerative changes could be seen in any of the treatment groups. In conclusion, both body AIMs and rotations were dose-dependently evoked by L-DOPA. Only AIMs, however, provided a specific measure of dyskinesia since rotations also were induced by bromocriptine, a drug with low dyskinesiogenic potential. Dose-failure episodes were not specific to L-DOPA treatment and could be attributed to erratic drug absorption from the peritoneal route.

Endothelial proliferation and increased blood-brain barrier permeability in the basal ganglia in a rat model of 3,4-dihydroxyphenyl-L-alanine-induced dyskinesia.

3,4-Dihydroxyphenyl-L-alanine (L-DOPA)-induced dyskinesia is associated with molecular and synaptic plasticity in the basal ganglia, but the occurrence of structural remodeling through cell genesis has not been explored. In this study, rats with 6-hydroxydopamine lesions received injections of the thymidine analog 5-bromo-2'-deoxyuridine (BrdU) concomitantly with L-DOPA for 2 weeks. A large number of BrdU-positive cells were found in the striatum and its output structures (globus pallidus, entopeduncular nucleus, and substantia nigra pars reticulata) in L-DOPA-treated rats that had developed dyskinesia. The vast majority (60-80%) of the newborn cells stained positively for endothelial markers. This endothelial proliferation was associated with an upregulation of immature endothelial markers (nestin) and a downregulation of endothelial barrier antigen on blood vessel walls. In addition, dyskinetic rats exhibited a significant increase in total blood vessel length and a visible extravasation of serum albumin in the two structures in which endothelial proliferation was most pronounced (substantia nigra pars reticulata and entopeduncular nucleus). The present study provides the first evidence of angiogenesis and blood-brain barrier dysfunction in an experimental model of L-DOPA-induced dyskinesia. These microvascular changes are likely to affect the kinetics of L-DOPA entry into the brain, favoring the occurrence of motor complications.

Role of striatal L-DOPA in the production of dyskinesia in 6-hydroxydopamine lesioned rats.

We explored possible differences in the peripheral and central pharmacokinetics of L-DOPA as a basis for individual variation in the liability to dyskinesia. Unilaterally, 6-hydroxydopamine (6-OHDA) lesioned rats were treated chronically with L-DOPA for an induction and monitoring of abnormal involuntary movements (AIMs). Comparisons between dyskinetic and non-dyskinetic cases were then carried out with regard to plasma and striatal L-DOPA concentrations, tissue levels of dopamine (DA), DA metabolites, and serotonin. After a single intraperitoneal injection of L-DOPA, plasma L-DOPA concentrations did not differ between dyskinetic and non-dyskinetic animals, whereas peak levels of L-DOPA in the striatal extracellular fluid were about fivefold larger in the former compared with the latter group. Interestingly, the time course of the AIMs paralleled the surge in striatal L-DOPA levels. Intrastriatal infusion of L-DOPA by reverse dialysis concentration dependently induced AIMs in all 6-OHDA lesioned rats, regardless of a previous priming for dyskinesia. Steady-state levels of DA and its metabolites in striatal and cortical tissue did not differ between dyskinetic and non-dyskinetic animals, indicating that the observed difference in motor response to L-DOPA did not depend on the extent of lesion-induced DA depletion. These results show that an elevation of L-DOPA levels in the striatal extracellular fluid is necessary and sufficient for the occurrence of dyskinesia. Individual differences in the central bioavailability of L-DOPA may provide a clue to the varying susceptibility to dyskinesia in Parkinson's disease.