Effects of levodopa on striatal monoamines in mice with levodopa-induced hyperactivity

Effects of acute and sub-chronic L-dopa therapy on striatal L-dopa methylation and dopamine oxidation in an MPTP mouse model of Parkinsons disease

AIMS

The molecular mechanisms for the loss of 3,4-dihydroxyphenylalanine (l-dopa) efficacy during the treatment of Parkinson's disease (PD) are unknown. Modifications related to catecholamine metabolism such as changes in l-dopa and dopamine (DA) metabolism, the modulation of catecholamine enzymes and the production of interfering metabolites are the primary concerns of this study.

MAIN METHODS

Normal (saline) and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) pre-treated mice were primed with 100mg/kg of l-dopa twice a day for 14 days, and a matching group remained l-dopa naïve. l-dopa naive and primed mice received a challenge dose of 100mg/kg of l-dopa and were sacrificed 30 min later. Striatal catecholamine levels and the expression and activity of catechol-O-methyltransferase (COMT) were determined.

KEY FINDINGS

Normal and MPTP pre-treated animals metabolize l-dopa and DA similarly during l-dopa therapy. Administration of a challenge dose of l-dopa increased l-dopa and DA metabolism in l-dopa naïve animals, and this effect was enhanced in l-dopa primed mice. The levels of 3-OMD in MPTP pre-treated animals were almost identical to those in normal mice, which we found are likely due to increased COMT activity in MPTP pre-treated mice.

SIGNIFICANCE

The results of this comparative study provide evidence that sub-chronic administration of l-dopa decreases the ability of the striatum to accumulate l-dopa and DA, due to increased metabolism via methylation and oxidation. This data supports evidence for the metabolic adaptation of the catecholamine pathway during long-term treatment with l-dopa, which may explain the causes for the loss of l-dopa efficacy.

Serotonergic pharmacology in animal models: from behavioral disorders to dyskinesia

Serotonin (5-HT) dysfunction has been involved in both movement and behavioral disorders. Serotonin pharmacology improves dyskinetic movements as well as depressive, anxious, aggressive and anorexic symptoms. Animal models have been useful to investigate more precisely to what extent 5-HT is involved and whether drugs targeting the 5-HT system can counteract the symptoms exhibited. We review existing rodent and non-human primate (NHP) animal models in which selective 5-HT or dual 5-HT-norepinephrine (NE) transporter inhibitors, as well as specific 5-HT receptors agonists and antagonists, monoamine oxidase A inhibitors (IMAO-A) and MDMA (Ecstasy) have been used. We review overlaps between the various drug classes involved. We confront behavioral paradigms and treatment regimen. Some but not all animal models and associated pharmacological treatments have been extensively studied in the litterature. In particular, the impact of selective serotonin reuptake inhibitors (SSRI) has been extensively investigated using a variety of pharmacological or genetic rodent models of depression, anxiety, aggressiveness. But the validity of these rodent models is questioned. On the contrary, few studies did address the potential impact of targeting the 5-HT system on NHP models of behavioral disorders, despite the fact that those models may match more closely to human pathologies. Further investigations with carefull behavioral analysis will improve our understanding of neural bases underlying the pathophysiology of movement and behavioral disorders.

Nonmotor symptoms in Parkinson's disease: expanding the view of Parkinson's disease beyond a pure motor, pure dopaminergic problem

Nonmotor symptoms (NMS) of Parkinson's disease (PD) are critical to identify and treat because of their impact on quality of life. Despite growing evidence of the importance of NMS on patients' quality of life, gaps remain in their recognition and treatment. The result is a need for increased information and understanding of specific NMS and the clinical approaches for their assessment and management in the context of PD as a whole. This article discusses the NMS of PD, their relationship to the pathologic basis of PD, and how NMS can be best managed.

Does MPTP intoxication in mice induce metabolite changes in the nucleus accumbens? A ¹H nuclear MRS study

Using in vivo ¹H NMR spectroscopy in a mouse model of Parkinson's disease, we previously showed that glutamate concentrations in the dorsal striatum were highest after dopamine denervation associated with an increase in gamma-aminobutyric acid (GABA) and (Gln) glutamine levels. The aim of this study was to determine whether the changes previously observed in the motor part of the striatum were reproduced in a ventral part of the striatum, the nucleus accumbens (NAc). This study was carried out on controls and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-intoxicated mice. In vivo spectra were acquired for a voxel (8 μL) in the dorsal striatum, and in the NAc (1.56 μL). NMR acquisitions were first performed 10 days after the last MPTP injection in a basal condition [after saline intraperitoneal (i.p.) injection] and then in the same animal the week after basal NMR acquisitions, after acute levodopa administration (200 mg kg⁻¹, i.p.). Immunohistochemistry was used to determine the levels of (Glu) glutamate, glutamine synthetase (GS) and glutamic acid decarboxylase (GAD) isoform 67 in these two structures. The Glu, Gln and GABA concentrations obtained in the basal state were higher in the NAc of MPTP-intoxicated mice which have the higher dopamine denervation in the ventral tegmental area (VTA) and in the dorsal striatum. Levodopa decreased the levels of these metabolites in MPTP-intoxicated mice to levels similar to those in controls. In parallel, immunohistochemical staining showed that glutamate, GS and GAD67 immunoreactivity increased in the dorsal striatum of MPTP-intoxicated mice and in the NAc for animals with a severe dopamine denervation in VTA. These findings strongly supported a hyperactivity of the glutamatergic cortico-striatal pathway and changes in glial activity when the dopaminergic denervation in the VTA and substantia nigra pars compacta (SNc) was severe.

L-Dihydroxyphenylalanine modulates the steady-state expression of mouse striatal tyrosine hydroxylase, aromatic L-amino acid decarboxylase, dopamine and its metabolites in an MPTP mouse model of Parkinson's disease

AIMS

l-3,4-Dihydroxyphenylalanine (L-DOPA) is the most effective symptomatic treatment for Parkinson's disease (PD), but PD patients usually experience a successful response to L-DOPA therapy followed by a progressive loss of response. L-DOPA efficacy relies on its decarboxylation by aromatic l-amino acid decarboxylase (AAAD) to form dopamine (DA). So exogenous L-DOPA drives the reaction and AAAD becomes the rate limiting enzyme in the supply of DA. In turn, exogenous L-DOPA regulates the expression and activity of AAAD as well as the synthesis of DA and its metabolites, changes that may be linked to the efficacy and side-effects of L-DOPA.

MAIN METHODS

One-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse PD model was utilized to study the effects of L-DOPA on the steady-state level and activity of AAAD, tyrosine hydroxylase (TH), DA and the metabolites of DA. The MPTP and control mice were treated twice daily with PBS or with 100mg/kg of L-DOPA for 14days and the expression and activity of AAAD, the expression of TH and the levels of DA and its metabolites were determined 24h after L-DOPA or PBS treatment, when exogenous L-DOPA is eliminated.

KEY FINDINGS

In the MPTP model, L-DOPA reduced the steady-state expression and the activity of striatal AAAD by 52% and 50%, respectively, DA and metabolites were also significantly decreased.

SIGNIFICANCE

The outcome shows that while L-DOPA replenishes striatal DA it also down-regulates AAAD and the steady-state synthesis and metabolic capability of the dopaminergic system. These findings are important in the precipitation of L-DOPA induced side effects and the management of L-DOPA therapy.

The roles of striatal serotonin and L -amino-acid decarboxylase on L-DOPA-induced Dyskinesia in a Hemiparkinsonian rat model

The administration of L: -DOPA is the standard treatment for Parkinson's disease (PD). However, the symptomatic relief provided by long-term administration may be compromised by L: -DOPA-induced dyskinesia (LID) that presents as adverse fluctuations in motor responsiveness and progressive loss of motor control. In the later stages of PD, raphestriatal serotonin neurons compensate for the loss of nigrostriatal dopamine (DA) neurons by converting and releasing DA derived from exogenous L: -DOPA. Since the serotonin system does not have an autoregulatory mechanism for DA, raphe-mediated striatal DA release may fluctuate dramatically and precede the development of LID. The 6-hydroxydopamine lesioned rats were treated with L: -DOPA (6 mg/kg) and benserazide (15 mg/kg) daily for 3 weeks to allow for the development of abnormal involuntary movement score (AIMs). In rats with LID, chronic treatment with L: -DOPA increased striatal DA levels compared with control rats. We also observed a relative increase in the expression of striatal L: -amino-acid decarboxylase (AADC) in LID rats, even though tyrosine hydroxylase (TH) expression did not increase. The administration of L: -DOPA also increased striatal serotonin immunoreactivity in LID rats compared to control rats. Striatal DA and 5-hydroxytryptamine (5-HT) levels were negatively correlated in L: -DOPA-treated rats. These results of this study reveal that 5-HT contributes to LID. Striatal DA positively influences LID, while 5-HT is negatively associated with LID. Finally, we suggest that by strategic modification of the serotonin system it may be possible to attenuate the adverse effects of chronic L: -DOPA therapy in PD patients.

Potential mechanisms underlying anxiety and depression in Parkinson's disease: consequences of l-DOPA treatment

Though the most recognizable symptoms of Parkinson's disease (PD) are motor-related, many patients also suffer from debilitating affective symptoms that deleteriously influence quality of life. Dopamine (DA) loss is likely involved in the onset of depression and anxiety in PD. However, these symptoms are not reliably improved by DA replacement therapy with l-3,4-dihydroxyphenylalanine (l-DOPA). In fact, preclinical and clinical evidence suggests that l-DOPA treatment may worsen affect. Though the neurobiological mechanisms remain unclear, recent research contends that l-DOPA further perturbs the function of the norepinephrine and serotonin systems, already affected by PD pathology, which have been intimately linked to the development and expression of anxiety and depression. As such, this review provides an overview of the clinical characteristics of affective disorders in PD, examines the utility of animal models for the study of anxiety and depression in PD, and finally, discusses potential mechanisms by which DA loss and subsequent l-DOPA therapy influence monoamine function and concomitant affective symptoms.

Metabolic changes detected in vivo by 1H MRS in the MPTP-intoxicated mouse

We used in vivo proton ((1)H) Magnetic Resonance Spectroscopy (MRS) to measure the levels of the main excitatory amino acid, glutamate (Glu) and also glutamine (Gln) and GABA in the striatum and cerebral cortex in the MPTP-intoxicated mouse, a model of dopaminergic denervation, before and after dopamine (DA) replacement. The study was performed at 9.4T on control mice (n = 8) and MPTP-intoxicated mice (n = 8). In vivo spectra were acquired in a voxel (8 microL) centered in the striatum, and in the cortex (4.6 microL). Three days after basal MRS acquisitions new spectra were acquired in the striatum and cortex, after levodopa (200 mg.kg(-1)). Glu, Gln and GABA concentrations obtained in the basal state were significantly increased in the striatum of MPTP-lesioned mice (Glu: 20.2 +/- 0.8 vs 11.4 +/- 0.9 mM, p < 0.001; Gln: 5.4 +/- 1.6 vs 2.0 +/- 0.6 mM, p < 0.05; GABA: 3.6 +/- 0.8 vs 1.6 +/- 0.2 mM, p < 0.05). Levodopa lowered metabolites concentrations in the striatum of MPTP-lesioned mice (Glu: 20.2 +/- 0.8 vs 11.2 +/- 0.4 mM (+ Ldopa), p < 0.001; Gln: 5.4 +/- 1.6 vs 1.6 +/- 0.4 mM (+ Ldopa), p < 0.05; GABA: 3.6 +/- 0.8 vs 1.7 +/- 0.4 mM (+ Ldopa), p < 0.01). Metabolite levels in the striatum of MPTP-intoxicated mice + levodopa were not significantly different from those in the striatum of controls. No change was found in the cortex after DA denervation and after DA replacement between the two animals groups. These results strongly support a predominant change in striatal Glu synaptic activity in the cortico-striatal pathway. Acute levodopa administration reverses the increase of metabolites in the striatum.