Repeated Administration Of L-DOPA Research Articles

Dopamine-Depleted Dopamine Transporter Knockout (DDD) Mice: Dyskinesia with L-DOPA and Dopamine D1 Agonists.

L-DOPA is the mainstay of treatment for Parkinson's disease (PD). However, over time this drug can produce dyskinesia. A useful acute PD model for screening novel compounds for anti-parkinsonian and L-DOPA-induced dyskinesia (LID) are dopamine-depleted dopamine-transporter KO (DDD) mice. Treatment with α-methyl-para-tyrosine rapidly depletes their brain stores of DA and renders them akinetic. During sensitization in the open field (OF), their locomotion declines as vertical activities increase and upon encountering a wall they stand on one leg or tail and engage in climbing behavior termed "three-paw dyskinesia". We have hypothesized that L-DOPA induces a stereotypic activation of locomotion in DDD mice, where they are unable to alter the course of their locomotion, and upon encountering walls engage in "three-paw dyskinesia" as reflected in vertical counts or beam-breaks. The purpose of our studies was to identify a valid index of LID in DDD mice that met three criteria: (a) sensitization with repeated L-DOPA administration, (b) insensitivity to a change in the test context, and (c) stimulatory or inhibitory responses to dopamine D1 receptor agonists (5 mg/kg SKF81297; 5 and 10 mg/kg MLM55-38, a novel compound) and amantadine (45 mg/kg), respectively. Responses were compared between the OF and a circular maze (CM) that did not hinder locomotion. We found vertical counts and climbing were specific for testing in the OF, while oral stereotypies were sensitized to L-DOPA in both the OF and CM and responded to D1R agonists and amantadine. Hence, in DDD mice oral stereotypies should be used as an index of LID in screening compounds for PD.

Rating L-DOPA-Induced Dyskinesias in the Unilaterally 6-OHDA-Lesioned Rat Model of Parkinson's Disease

L-DOPA-induced dyskinesias (LIDs) refer to motor complications that arise from prolonged L-DOPA administration to patients with Parkinson's disease (PD). The most common pattern observed in the clinic is the peak-dose dyskinesia which consists of clinical manifestations of choreiform, dystonic, and ballistic movements. The 6-hydroxydopamine (6-OHDA) rat model of PD mimics several characteristics of LIDs. After repeated L-DOPA administration, 6-OHDA-lesioned rats exhibit dyskinetic-like movements (e.g., abnormal involuntary movements, AIMs). This protocol demonstrates how to induce and analyze AIMs in 6-OHDA-lesioned rats with 90%-95% dopaminergic depletion in the nigrostriatal pathway. Repeated administration (3 weeks) of L-DOPA (5 mg/kg, combined with 12.5 mg/kg of benserazide) can induce the development of AIMs. The time course analysis reveals a significant increase in AIMs at 30-90 min (peak-dose dyskinesia). Rodent models of LIDs are an important preclinical tool to identify effective antidyskinetic interventions.

Rating L-DOPA-Induced Dyskinesias in the Unilaterally 6-OHDA-Lesioned Rat Model of Parkinson's Disease.

L-DOPA-induced dyskinesias (LIDs) refer to motor complications that arise from prolonged L-DOPA administration to patients with Parkinson's disease (PD). The most common pattern observed in the clinic is the peak-dose dyskinesia which consists of clinical manifestations of choreiform, dystonic, and ballistic movements. The 6-hydroxydopamine (6-OHDA) rat model of PD mimics several characteristics of LIDs. After repeated L-DOPA administration, 6-OHDA-lesioned rats exhibit dyskinetic-like movements (e.g., abnormal involuntary movements, AIMs). This protocol demonstrates how to induce and analyze AIMs in 6-OHDA-lesioned rats with 90%-95% dopaminergic depletion in the nigrostriatal pathway. Repeated administration (3 weeks) of L-DOPA (5 mg/kg, combined with 12.5 mg/kg of benserazide) can induce the development of AIMs. The time course analysis reveals a significant increase in AIMs at 30-90 min (peak-dose dyskinesia). Rodent models of LIDs are an important preclinical tool to identify effective antidyskinetic interventions.

L-DOPA-elicited abnormal involuntary movements in the rats damaged severely in substantia nigra by 6-hydroxydopamine.

Dyskinesia of rat models can occur in several conditions: acute levodopa (L-DOPA) administration provided that the drug dose is sufficiently high and/or that the nigrostriatal dopamine (DA) pathway is seriously damaged, and repeated L-DOPA administration which could cause a reduction of the dyskinesia-threshold dose, a progressive aggravation and an increasing incidence of dyskinesia. Therefore, if the damage of the nigrostriatal DA pathway is extremely severe, what abnormal movements can be elicited by first injecting L-DOPA or other dopaminergic agonists? The problem deserves exploring. Rat models with damage of varying severity were divided into three groups: the serious lesion [induced by 40 µg 6-hydroxydopamine (6-OHDA), two injected coordinates including substantia nigra (SN) and medial forebrain bundle], the moderate lesion (20 µg 6-OHDA, a coordinate in SN) and the control. Three weeks after lesion, the Rota Rad test and Cylinder test were performed to assess the motor activities of rat models, the abnormal involuntary movements (AIMs) elicited by L-DOPA or apomorphine (APO) were observed, and the dopaminergic degeneration in SN and striatum was determined. Both seriously lesioned rats and the moderately were observed to exhibit a significant decrease in motor activities. In the rats with a serious lesion, scarcely any dopaminergic neurons were present in the SN, tissue DA level decreased by 99% in the striatum, and both L-DOPA and APO could elicit AIMs and rotational movements. In the rats with the moderate lesion, only rotation movements could be elicited. The rotation speed of moderately lesioned rats was 9 turns/min, but that of seriously was only 4.5 turns/min elicited by APO. Both dyskinesia and rotation movement are the specific expressions elicited by L-DOPA or APO in rats whose SN is damaged by 6-OHDA. Dyskinesia reflects more severe damage than rotation movement.

The novel 5-HT1A receptor agonist, NLX-112 reduces l-DOPA-induced abnormal involuntary movements in rat: A chronic administration study with microdialysis measurements

Although l-DOPA alleviates the motor symptoms of Parkinson's disease (PD), it elicits troublesome l-DOPA-induced dyskinesia (LID) in a majority of PD patients after prolonged treatment. This is likely due to conversion of l-DOPA to dopamine as a ‘false neurotransmitter’ from serotoninergic neurons. The highly selective and efficacious 5-HT1A receptor agonist, NLX-112 (befiradol or F13640) shows potent activity in a rat model of LID (suppression of Abnormal Involuntary Movements, AIMs) but its anti-AIMs effects have not previously been investigated following repeated administration.Acute administration of NLX-112 (0.04 and 0.16 mg/kg i.p.) reversed l-DOPA (6 mg/kg)-induced AIMs in hemiparkinsonian rats with established dyskinesia. The activity of NLX-112 was maintained following repeated daily i.p. administration over 14 days and was accompanied by pronounced decrease of striatal 5-HT extracellular levels, as measured by in vivo microdialysis, indicative of the inhibition of serotonergic activity. A concurrent blunting of l-DOPA-induced surge in dopamine levels on the lesioned side of the brain was observed upon NLX-112 administration and these neurochemical responses were also seen after 14 days of treatment.NLX-112 also suppressed the expression of AIMs in rats that were being primed for dyskinesia by repeated l-DOPA administration. However, when treatment of these rats with NLX-112 was stopped, l-DOPA then induced AIMs with scores that resembled those of control rats.The present study shows that the potent anti-AIMs activity of NLX-112 is maintained upon repeated administration and supports the ongoing clinical development of NLX-112 as a novel antidyskinetic agent for PD patients receiving l-DOPA treatment.

Intranigral administration of substance P receptor antagonist attenuated levodopa-induced dyskinesia in a rat model of Parkinson's disease

Levodopa (l-dopa) remains the most effective drug in the treatment of Parkinson's disease (PD). However, l-dopa-induced dyskinesia (LID) has hindered its use for PD patients. The mechanisms of LID are not fully understood. Substance P (SP) receptor antagonist has been shown to reduce parkinsonism in animal models of PD, and ameliorate LID in PD rats. But the concrete mechanism is not fully understood. To address this issue, we produced a rat model of PD using 6-hydroxydompamine (6-OHDA) injections, and valid PD rats were intranigrally administrated with different doses of SP receptor antagonist LY303870 (5nmol/day, 10nmol/day and 20nmol/day) following l-dopa (6mg/kg/day, i.p.) plus benserazide (12mg/kg/day, i.p.) for 23days. We found that nigral SP levels were increased on days 3, 7 and 14 and decreased on day 21 after 6-hydroxydompamine lesions. But nigral SP levels kept increasing after repeated l-dopa administration in PD rats. Intranigral administration of low and moderate LY303870 reduced abnormal involuntary movements (AIMs) while improving motor deficits in PD rats treated with l-dopa plus benserazide. Microdialysis revealed that LY303870 (10nmol/day) treatment attenuated the increase of striatal dopamine and the reduction of γ-aminobutyric acid in ventromedial thalamus of PD rats primed with l-dopa. Additionally, LY303870 (10nmol/day) treatment prior to l-dopa administration reduced the phosphorylated levels of dopamine- and cyclic adenosine monophosphate-regulated phosphoprotein of 32kDa at Thr 34 and extracellular signal-regulated kinases 1/2 as well as the levels of activity-regulated cytoskeleton-associated protein and Penk in l-dopa-primed PD rats. Taken together, these data showed that low and moderate SP receptor antagonists LY303870 could ameliorate LID via neurokinin 1 receptor without affecting therapeutic effect of l-dopa.

L-DOPA-induced behavioral sensitization of motor activity in the MPTP-treated common marmoset as a Parkinson's disease model

l-DOPA is the gold standard for treatment of Parkinson's disease (PD). However, the drug produces some serious side effects, including dyskinesia, which is characterized by repetitive involuntary movements—including chorea. In the present preclinical study using a nonhuman primate model, dyskinesia caused by repeated l-DOPA administration was investigated in the context of behavioral sensitization by objectively quantifying motor activity in the common marmoset of PD model (the Parkinsonian marmoset). Twelve male Parkinsonian marmosets previously treated with MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) and six intact male marmosets were used. The motor activity of each marmoset was measured using infrared sensors attached to each individual living cage. Parkinsonian marmosets (n=6) exhibited behavioral sensitization (enhanced motor activity) in 10weeks upon oral administration of l-DOPA (10mg/kg per day on 3days/week). These animals also exhibited dyskinesia characterized by repetitive rapid movements including chorea in 6–10weeks. Neither behavioral sensitization nor dyskinesia was observed in Parkinsonian marmosets given vehicle and in intact marmosets given l-DOPA at the same dose (both n=6 each). Behavioral sensitization was detected sensitively and objectively on motor activity only in Parkinsonian marmosets given repeated l-DOPA at a similar dose used in PD patients. The behavioral feature of the marmosets was dyskinesia similar to that of PD patients but appeared earlier than would be manifested in humans. In spite of statistically significant behavioral sensitization, some marmosets did not exhibit dyskinesia in the present limited l-DOPA administration period. Although both commonalities and differences may exist between behavioral sensitization and dyskinesia, behavioral sensitization is considered to be an objective, quantitative, sensitive and predictive measure of behavioral mechanism underlying dyskinesia in preclinical studies in evaluating compounds.

Striatal astrocytes act as a reservoir for L-DOPA.

L-DOPA is therapeutically efficacious in patients with Parkinson’s disease (PD), although dopamine (DA) neurons are severely degenerated. Since cortical astrocytes express neutral amino acid transporter (LAT) and DA transporter (DAT), the uptake and metabolism of L-DOPA and DA in striatal astrocytes may influence their availability in the dopaminergic system of PD. To assess possible L-DOPA- and DA-uptake and metabolic properties of striatal astrocytes, we examined the expression of L-DOPA, DA and DAT in striatal astrocytes of hemi-parkinsonian model rats after repeated L-DOPA administration, and measured the contents of L-DOPA, DA and their metabolite in primary cultured striatal astrocytes after L-DOPA/DA treatment. Repeated injections of L-DOPA induced apparent L-DOPA- and DA-immunoreactivities and marked expression of DAT in reactive astrocytes on the lesioned side of the striatum in hemi-parkinsonian rats. Exposure to DA for 4h significantly increased the levels of DA and its metabolite DOPAC in cultured striatal astrocytes. L-DOPA was also markedly increased in cultured striatal astrocytes after 4-h L-DOPA exposure, but DA was not detected 4 or 8h after L-DOPA treatment, despite the expression of aromatic amino acid decarboxylase in astrocytes. Furthermore, the intracellular level of L-DOPA in cultured striatal astrocytes decreased rapidly after removal of extracellular L-DOPA. The results suggest that DA uptaken into striatal astrocytes is rapidly metabolized and that striatal astrocytes act as a reservoir of L-DOPA that govern the uptake or release of L-DOPA depending on extracellular L-DOPA concentration, but are less capable of converting L-DOPA to DA.

Dyskinetic patients show rebound worsening of affect after an acute L-dopa challenge

BackgroundMotor response complications that arise with repeated L-dopa administration for the treatment of Parkinson’s disease are well understood but the relationship between motor response complications and affect are not. We proposed that patients with dyskinesias would report rebound worsening in affect during wearing-off of L-dopa effect. MethodsFifty Parkinson’s disease patients with were assessed with the Purdue Pegboard test and rated Positive Affect and Negative Affect after overnight withdrawal of dopaminergic medications and half hourly for 6 h after a standard L-dopa challenge. Patients were carefully classified into stable responder (n = 12), fluctuator (n = 15), and dyskinetic (n = 23) groups. ResultsPositive Affect was improved by L-dopa in dyskinetics and to a lesser degree in fluctuators but not in stable responders. At T = 4–6 h, Positive Affect rebounded below baseline in dyskinetics only. On regression analysis, rebound worsening positively correlated with ratings of dyskinesia severity. Negative Affect improved with L-dopa in all groups and tended to remain below baseline for 6 h after L-dopa challenge. Peak effects of L-dopa on Positive Affect and Negative Affect occurred significantly earlier than effects on Purdue Pegboard test and were positively correlated with L-dopa equivalent daily dose. ConclusionThere is a clinical dissociation between L-dopa effects on motor function, Positive Affect and Negative Affect. Rebound worsening in Positive Affect occurred only in dyskinetic patients and the onset of rebound worsening occurred before the end of the motor benefit phase. These observations could explain why some Parkinson patients report wearing-off symptoms despite the external impression of good motor control.

Effect of Adenosine A2A Receptor Antagonists and l-DOPA on Hydroxyl Radical, Glutamate and Dopamine in the Striatum of 6-OHDA-Treated Rats

A2A adenosine receptor antagonists have been proposed as a new therapy of PD. Since oxidative stress plays an important role in the pathogenesis of PD, we studied the effect of the selective A2A adenosine receptor antagonists 8-(-3-chlorostyryl)caffeine (CSC) and 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM 241385) on hydroxyl radical generation, and glutamate (GLU) and dopamine (DA) extracellular level using a microdialysis in the striatum of 6-OHDA-treated rats. CSC (1 mg/kg) and ZM 241385 (3 mg/kg) given repeatedly for 14 days decreased the production of hydroxyl radical and extracellular GLU level, both enhanced by prior 6-OHDA treatment in dialysates from the rat striatum. CSC and ZM 241385 did not affect DA and its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanilic acid (HVA) extracellular levels in the striatum of 6-OHDA-treated rats. l-DOPA (6 mg/kg) given twice daily for two weeks in the presence of benserazide (3 mg/kg) decreased striatal hydroxyl radical and glutamate extracellular level in 6-OHDA-treated rats. At the same time, l-DOPA slightly but significantly increased the extracellular levels of DOPAC and HVA. A combined repeated administration of l-DOPA and CSC or ZM 241385 did not change the effect of l-DOPA on hydroxyl radical production and glutamate extracellular level in spite of an enhancement of extracellular DA level by CSC and elevation of extracellular level of DOPAC and HVA by ZM 241385. The data suggest that the 6-OHDA-induced damage of nigrostriatal DA-terminals is related to oxidative stress and excessive release of glutamate. Administration of l-DOPA in combination with CSC or ZM 241385, by restoring striatal DA-glutamate balance, suppressed 6-OHDA-induced overproduction of hydroxyl radical.

Induction and expression of abnormal involuntary movements is related to the duration of dopaminergic stimulation in 6‐OHDA‐lesioned rats

Dyskinesia induction in Parkinson's disease (PD) appears less marked with long-acting dopamine agonists than with short-acting L-Dopa, but the relationship to duration of drug action is unknown. It is also unclear whether the duration of drug action affects the expression of established dyskinesia. This study compared the ability of L-Dopa and four dopamine agonists of different duration of action to induce abnormal involuntary movements (AIMs) in 6-hydroxydopamine (6-OHDA)-lesioned rats, and their ability to express established AIMs following prior exposure to L-Dopa. 6-OHDA-lesioned rats were treated with saline, L-Dopa/benserazide, apomorphine, ropinirole, pramipexole or pergolide once daily for 15 days. Repeated administration of the short-acting dopamine agonists, apomorphine (duration 80 min) and ropinirole (duration 90 min) induced marked axial, limb and orolingual AIMs at peak effect. L-Dopa (duration 100 min) produced moderate AIMs at peak effect, while administration of the long-acting dopamine agonists, pramipexole (duration 150 min) and pergolide (duration 240 min) resulted in mild AIMs. In rats primed to exhibit severe AIMs following repeated L-Dopa administration, acute administration of apomorphine, ropinirole and L-Dopa induced severe AIMs. By contrast, pramipexole and pergolide evoked only mild-moderate AIMs. Again, there was a negative correlation between duration of effect and the severity of AIMs expressed. These studies show that both the induction and expression of AIMs in 6-OHDA-lesioned rats are related to the duration of action of dopaminergic drugs. These findings suggest that continuous dopaminergic stimulation could be used both to avoid dyskinesia induction and to improve motor function in late-stage PD when troublesome dyskinesia is evident.

Pre-treatment with dopamine agonists influence l-dopa mediated rotations without affecting abnormal involuntary movements in the 6-OHDA lesioned rat

l-dopa induced dyskinesia is a complication of long-term l-dopa administration in patients with Parkinson's disease. This study uses the rodent model of dyskinesia to determine whether prior dopamine agonist treatment causes long-term changes that influence the development of l-dopa mediated behaviours. Rats with unilateral 6-OHDA lesions were injected with dopamine agonists (ropinirole, piribedil bromocriptine, all 1 mg/kg) or saline (0.9%) daily for 21 days. Following a 1-week drug free interval l-dopa was administered for 15 days (10 mg/kg with benserazide 15 mg/kg in saline s.c.). Rotational behaviour and abnormal involuntary movements (AIMs) were recorded at regular intervals. All dopamine agonists induced a contralateral rotational response on day 1, which increased in response to repeated administration but did not by themselves induce overt dyskinesias. On day 1 of l-dopa administration animals pre-treated with piribedil and ropinirole produced a more severe rotational response. In the saline pre-treated group, AIMs developed with repeated l-dopa administration, which was reflected in the increased expression of PPE-B mRNA. There was a trend for the same pattern in the dopamine agonist treated groups but this was non-significant. Therefore, while locomotor sensitivity is altered by the pre-treatment with dopamine agonists, there appears to be no increased risk of developing AIMs.

Intermittent l-DOPA treatment differentially alters synaptotagmin 4 and 7 gene expression in the striatum of hemiparkinsonian rats

Long-term use of l-DOPA in Parkinson's disease (PD) is frequently associated with side effects that are reflected in changed neurotransmitter/neuropeptide secretion in basal ganglia. These side effects could be connected with synaptotagmins (syts) because syts are involved in regulation of membrane trafficking. We have previously reported that acute l-DOPA treatment upregulated the expression of Syt 4 and Syt 7 mRNAs in hypersensitive striatum of 6-OHDA rat model for PD. Here we investigate whether intermittent l-DOPA treatment that produces behavior sensitization affects the Syt 1, Syt 2, Syt 4, Syt 7 and Syt 10 mRNAs in striatum of 6-OHDA rats killed 4 and 12 h after the last l-DOPA injection. We verified behavioral sensitization by increased intensity of contralateral turning. 6-OHDA lesion caused Syt 2 mRNA downregulation and Syt 10 mRNA upregulation in striatum, but failed to alter Syt 4, Syt 7 and Syt 1 mRNAs. Acute l-DOPA induced an increase of Syt 4 and Syt 7 mRNAs in the denervated striatum leaving the levels of Syt 1, Syt 2 and Syt 10 mRNAs unaffected. Intermittent l-DOPA treatment did not alter Syt 1, Syt 2 and Syt 10 mRNA striatal levels, suggesting that 6-OHDA-induced Syt 2 and Syt 10 mRNA changes reflect a persistent striatal abnormality caused by dopamine depletion. On contrary, intermittent l-DOPA treatment downregulated Syt 4 mRNA and prolonged the elevation of Syt 7 mRNA in the denervated striatum. We conclude that Syt 4 and Syt 7 might be specifically involved in striatal plasticity caused by repeated l-DOPA administration that accompanies sensitization.

Receptor‐activity modifying protein 1 expression is increased in the striatum following repeated L‐DOPA administration in a 6‐hydroxydopamine lesioned rat model of Parkinson's disease

Abnormalities in signaling by G-protein coupled receptors (GPCRs) within the striatum are involved in the pathophysiology of Parkinson's disease (PD) and L-DOPA induced dyskinesia (LID). Receptor activity modifying proteins (RAMPs) are single transmembrane accessory proteins crucial for both trafficking and defining the phenotype of GPCRs. In the CNS, RAMP1 mRNA is predominantly expressed in striatum, cortex, and olfactory tubercles. In the present study, expression of RAMP1 mRNA is increased in the striatum (68-77%), following repeated L-DOPA administration, in the 6-hydroxydopamine-lesioned rat. These data are the first to describe regulation of RAMP1 expression in the CNS and suggest that changes in RAMP1 activity are involved in the pathophysiology of LID.

Protective effects of metallothionein against dopamine quinone-induced dopaminergic neurotoxicity

Dopamine (DA) quinone as DA neuron-specific oxidative stress conjugates with cysteine residues in functional proteins to form quinoproteins. Here, we examined the effects of cysteine-rich metal-binding proteins, metallothionein (MT)-1 and -2, on DA quinone-induced neurotoxicity. MT quenched DA semiquinones in vitro. In dopaminergic cells, DA exposure increased quinoproteins and decreased cell viability; these were ameliorated by pretreatment with MT-inducer zinc. Repeated L-DOPA administration markedly elevated striatal quinoprotein levels and reduced the DA nerve terminals specifically on the lesioned side in MT-knockout parkinsonian mice, but not in wild-type mice. Our results suggested that intrinsic MT protects against L-DOPA-induced DA quinone neurotoxicity in parkinsonian mice by its quinone-quenching property.

Opioids and motor complications in Parkinson's disease

The long-term treatment of Parkinson's disease with L-dopa is often associated with the appearance of involuntary movements called L-dopa-induced dyskinesias. These debilitating side-effects are thought to result from an aberrant form of plasticity triggered by a combination of factors related to dopamine denervation and repeated L-dopa administration. In animal models of Parkinson's disease, dopamine denervation and repeated L-dopa administration are associated with an enhancement of opioid transmission in the basal ganglia. The exact role of this increased opioid activity is still under debate. It has been proposed that some of the changes in opioid transmission are directly involved in the genesis of L-dopa-induced dyskinesias. In this article, we suggest that changes in opioid transmission in the basal ganglia in response to denervation and repeated L-dopa therapy are, instead, part of compensatory mechanisms to prevent motor complications. Initially, these compensatory mechanisms might be sufficient to attenuate the parkinsonian syndrome and delay the appearance of involuntary movements. But with the progression of the disease and repeated exposure to L-dopa, these mechanisms eventually fail. These new insights could contribute to better understanding of the motor complications in Parkinson's disease and lead to the development or improvement of pharmacological strategies to prevent or reduce L-dopa-induced dyskinesias.

Repeated administration of the monoamine reuptake inhibitor BTS 74 398 induces ipsilateral circling in the 6‐hydroxydopamine lesioned rat without sensitizing motor behaviours

BTS 74 398 (1-[1-(3,4-dichlorophenyl)cyclobutyl]-2-(3-diaminethylaminopropylthio)ethanone monocitrate) is a monoamine reuptake inhibitor that reverses motor deficits in MPTP-treated (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) common marmosets without provoking established dyskinesia. However, it is not known whether BTS 74 398 primes the basal ganglia for dyskinesia induction. In this study, the ability of BTS 74 398 to sensitize 6-hydroxydopamine (6-OHDA)-lesioned rats for the production of abnormal motor behaviours and the induction of striatal DeltaFosB were determined in comparison with l-3,4-dihydroxyphenylalanine methyl ester (L-dopa). Acute administration of BTS 74 398 induced a dose-dependent ipsilateral circling response in unilaterally 6-OHDA-lesioned rats whereas L-dopa produced dose-dependent contraversive rotation. The ipsilateral circling response to BTS 74 398 did not alter during 21 days of administration. In contrast, L-dopa treatment for 21 days caused a marked increase in rotational response. Repeated administration of both L-dopa and BTS 74 398 increased general motor activity and stereotypic behaviour. In L-dopa-treated rats, orolingual, locomotive, forelimb and axial abnormal movements developed whereas BTS 74 398 produced only locomotion with a side bias but no other abnormal movements. Sensitization of circling responses and the development of abnormal movements in 6-OHDA-lesioned rats have been associated with the potential of dopaminergic drugs to induce dyskinesia. Furthermore, striatal DeltaFosB immunoreactivity, shown to correlate with dyskinesia induction, was increased by L-dopa but was unaffected by repeated BTS 74 398 administration. The lack of such changes following repeated BTS 74 398 treatment suggests that it may be an effective antiparkinsonian therapy that is unlikely to produce involuntary movements.

The MPTP-treated primate as a model of motor complications in PD: primate model of motor complications.

Motor complications are a common cause of treatment failure in Parkinson's disease (PD). Although the underlying mechanisms remain obscure, research with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated primates may facilitate their investigation. Repeated administration of L-dopa to MPTP-treated primates results in a loss of drug efficacy ("wearing off") and periods of immobility ("on-off"). Immediately after each dose of L-dopa, animals show an initial worsening of motor performance ("beginning-of-dose deterioration") and, at the end of action of each dose, they exhibit a decline to levels of disability below those seen at baseline ("rebound worsening"). Dyskinesia (chorea, dystonia, athetosis) appears rapidly in MPTP-treated primates given L-dopa. The greater the degree of nigral denervation, the less exposure to L-dopa that is required to induce dyskinesia. The onset and intensity of dyskinesia are related to the dose and frequency of L-dopa administration. Increasing brain exposure to L-dopa increases the severity and intensity of involuntary movements. In contrast, long-acting dopamine agonists (DAs) induce a far lower incidence of dyskinesia, and continuous dopaminergic stimulation may provide a means of avoiding its onset. Recent data from MPTP-treated primates suggest that switching from L-dopa to a long-acting DA may reverse the priming process associated with induction of dyskinesia. Responses of MPTP-treated primates have proved to be highly predictive of antiparkinsonian activities of drugs in humans and their ability to induce dyskinesia, but they may also have utility in evaluating the mechanisms that underlie a range of long-term motor complications affecting patients with PD.

Denervation and repeated l-DOPA induce a coordinate expression of the transcription factor NGFI-B in striatal projection pathways in hemi-parkinsonian rats

Repeated intermittent administration of l-DOPA in rats with a unilateral 6-hydroxydopamine (6-OHDA) lesion of the nigrostriatal pathway results in a progressive increase of contraversive circling behavior. In this study, we have investigated the effects of denervation and repeated l-DOPA administration on the expression of the nuclear receptor nerve growth factor inducible-B (NGFI-B) in striatal output pathways of unilaterally 6-OHDA-lesioned rats. The denervation process induced an increase of NGFI-B and enkephalin (ENK) mRNA levels and the increase of NGFI-B took place predominantly in ENK-containing cells. The percentage of cells colocalizing NGFI-B and dynorphin (DYN) was significantly reduced. Repeated l-DOPA treatment increased the striatal level of DYN mRNA but it further reduced the percentage of NGFI-B/DYN double-labeled cells. On the intact side, repeated l-DOPA treatment increased NGFI-B expression in both striatal subpopulations. Additional acute studies were performed in normal rats to determine the role of the denervation process in the coordinate expression of NGFI-B in striatal subpopulations. A combination of selective D 1 and D 2 agonists induced an important increase of striatal NGFI-B expression selectively in DYN-containing neurons. These results demonstrate that the denervation process causes a differential regulation of NGFI-B in the two striatal output pathways which is further exacerbated by l-DOPA treatment. These molecular changes in response to dopamine depletion and dopamine replacement therapy may contribute to the long-term effects of l-DOPA and to the development of behavioral sensitization.

Effect of pulsatile administration of levodopa on dyskinesia induction in drug-naïve MPTP-treated common marmosets: effect of dose, frequency of administration, and brain exposure.

Levodopa (L-dopa) consistently primes basal ganglia for the appearance of dyskinesia in parkinsonian patients and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine hydrochloride (MPTP) -treated primates. This finding may reflect its relatively short duration of effects resulting in pulsatile stimulation of postsynaptic dopamine receptors in the striatum. We have compared the relationship between L-dopa dose and frequency of administration on dyskinesia initiation in drug-naïve, MPTP-treated common marmosets. We have also studied the effect of increased brain exposure to pulsatile administration by combining a low-dose of L-dopa with the peripheral catechol-O-methyltransferase inhibitor (COMT-I), entacapone. Pulsatile administration of a low (dose range, 5.0-7.5 mg/kg p.o.) or a high (12.5 mg/kg) dose of L-dopa plus carbidopa b.i.d. produced a dose-related reversal of motor deficits. Repeated administration of low and high doses of L-dopa for 26 days to drug-naïve, MPTP-treated animals also caused a dose-related induction of peak-dose dyskinesia. Repeated administration of high-dose L-dopa b.i.d. compared to once daily caused a frequency-related improvement of motor symptoms, resulting in a more rapid and initially more intense appearance of peak-dose dyskinesia. Administration of low-dose L-dopa b.i.d. for 26 days in combination with entacapone enhanced the increase in locomotor activity and reversal of disability produced by L-dopa alone, but with no obvious change in duration of L-dopa's effect. However, combining entacapone with L-dopa resulted in the more rapid appearance of dyskinesia, which was initially more severe than occurred with L-dopa alone. Importantly, increasing pulsatile exposure of brain to L-dopa by preventing its peripheral breakdown also increases dyskinesia induction.