l-Dopa induced dyskinesias in Parkinsonian mice: Disease severity or l-Dopa history

AbstractIn Parkinson’s disease, the efficacy of l-Dopa treatment changes over time, as dyskinesias emerge with previously beneficial doses. Using MitoPark mice, that models mitochondrial failure in dopamine (DA) neurons and mimics the progressive loss of dopamine observed in Parkinson’s disease, we found that the severity of DA denervation and associated adaptations in striatal neurotransmission at the time of initiation of l-Dopa treatment determines development of l-Dopa induced dyskinesias. We treated 20-week, and 28-week old MitoPark mice with l-Dopa (10mg/kg i.p. twice a day) and found locomotor responses to be significantly different. While all MitoPark mice developed sensitization to l-Dopa treatment over time, 28-week old MitoPark mice with extensive striatal DA denervation developed abnormal involuntary movements rapidly and severely after starting l-Dopa treatment, as compared to a more gradual escalation of movements in 20-week old animals that started treatment at earlier stages of degeneration. Our data support that it is the extent of loss of DA innervation that determines how soon motor complications develop with l-Dopa treatment. Gene array studies of striatal neurotransmitter receptors revealed changes in mRNA expression levels for DA, serotonin, glutamate and GABA receptors in striatum of 28-week old MitoPark mice. Our results support that delaying l-Dopa treatment until Parkinson’s disease symptoms become more severe does not delay the development of l-Dopa-induced dyskinesias. MitoPark mice model genetic alterations known to impair mitochondrial function in a subgroup of Parkinson patients and provide a platform in which to study treatments to minimize the development of dyskinesia

NMDA Receptors on Non-Dopaminergic Neurons in the VTA Support Cocaine Sensitization

The initiation of behavioral sensitization to cocaine and other psychomotor stimulants is thought to reflect N-methyl-D-aspartate receptor (NMDAR)-mediated synaptic plasticity in the mesolimbic dopamine (DA) circuitry. The importance of drug induced NMDAR mediated adaptations in ventral tegmental area (VTA) DA neurons, and its association with drug seeking behaviors, has recently been evaluated in Cre-loxp mice lacking functional NMDARs in DA neurons expressing Cre recombinase under the control of the endogenous dopamine transporter gene (NR1(DATCre) mice).Using an additional NR1(DATCre) mouse transgenic model, we demonstrate that while the selective inactivation of NMDARs in DA neurons eliminates the induction of molecular changes leading to synaptic strengthening, behavioral measures such as cocaine induced locomotor sensitization and conditioned place preference remain intact in NR1(DATCre) mice. Since VTA DA neurons projecting to the prefrontal cortex and amygdala express little or no detectable levels of the dopamine transporter, it has been speculated that NMDA receptors in DA neurons projecting to these brain areas may have been spared in NR1(DATCre) mice. Here we demonstrate that the NMDA receptor gene is ablated in the majority of VTA DA neurons, including those exhibiting undetectable DAT expression levels in our NR1(DATCre) transgenic model, and that application of an NMDAR antagonist within the VTA of NR1(DATCre) animals still blocks sensitization to cocaine.These results eliminate the possibility of NMDAR mediated neuroplasticity in the different DA neuronal subpopulations in our NR1(DATCre) mouse model and therefore suggest that NMDARs on non-DA neurons within the VTA must play a major role in cocaine-related addictive behavior

Selective Deletion of PTEN in Dopamine Neurons Leads to Trophic Effects and Adaptation of Striatal Medium Spiny Projecting Neurons

The widespread distribution of the tumor suppressor PTEN in the nervous system suggests a role in a broad range of brain functions. PTEN negatively regulates the signaling pathways initiated by protein kinase B (Akt) thereby regulating signals for growth, proliferation and cell survival. Pten deletion in the mouse brain has revealed its role in controlling cell size and number. In this study, we used Cre-loxP technology to specifically inactivate Pten in dopamine (DA) neurons (Pten KO mice). The resulting mutant mice showed neuronal hypertrophy, and an increased number of dopaminergic neurons and fibers in the ventral mesencephalon. Interestingly, quantitative microdialysis studies in Pten KO mice revealed no alterations in basal DA extracellular levels or evoked DA release in the dorsal striatum, despite a significant increase in total DA tissue levels. Striatal dopamine receptor D1 (DRD1) and prodynorphin (PDyn) mRNA levels were significantly elevated in KO animals, suggesting an enhancement in neuronal activity associated with the striatonigral projection pathway, while dopamine receptor D2 (DRD2) and preproenkephalin (PPE) mRNA levels remained unchanged. In addition, PTEN inactivation protected DA neurons and significantly enhanced DA-dependent behavioral functions in KO mice after a progressive 6OHDA lesion. These results provide further evidence about the role of PTEN in the brain and suggest that manipulation of the PTEN/Akt signaling pathway during development may alter the basal state of dopaminergic neurotransmission and could provide a therapeutic strategy for the treatment of Parkinson's disease, and other neurodegenerative disorders

Attenuated response to methamphetamine sensitization and deficits in motor learning and memory after selective deletion of β-catenin in dopamine neurons

In the present study, we analyzed mice with a targeted deletion of β-catenin in DA neurons (DA-βcat KO mice) to address the functional significance of this molecule in the shaping of synaptic responses associated with motor learning and following exposure to drugs of abuse. Relative to controls, DA-βcat KO mice showed significant deficits in their ability to form long-term memories and displayed reduced expression of methamphetamine- induced behavioral sensitization after subsequent challenge doses with this drug, suggesting that motor learning and drug-induced learning plasticity are altered in these mice. Morphological analyses showed no changes in the number or distribution of tyrosine hydroxylase-labeled neurons in the ventral midbrain. While electrochemical measurements in the striatum determined no changes in acute DA release and uptake, a small but significant decrease in DA release was detected in mutant animals after prolonged repetitive stimulation, suggesting a possible deficit in the DA neurotransmitter vesicle reserve pool. However, electron microscopy analyses did not reveal significant differences in the content of synaptic vesicles per terminal, and striatal DA levels were unchanged in DA-βcat KO animals. In contrast, striatal mRNA levels for several markers known to regulate synaptic plasticity and DA neurotransmission were altered in DA-βcat KO mice. This study demonstrates that ablation of β-catenin in DA neurons leads to alterations of motor and reward-associated memories and to adaptations of the DA neurotransmitter system and suggests that β-catenin signaling in DA neurons is required to facilitate the synaptic remodeling underlying the consolidation of long-term memories. © 2012 Cold Spring Harbor Laboratory Press.Fil: Diaz-Ruiz, Oscar. National Institutes of Health; Estados UnidosFil: Zhang, YaJun. National Institutes of Health; Estados UnidosFil: Shan, Lufei. National Institutes of Health; Estados UnidosFil: Malik, Nasir. National Institutes of Health; Estados UnidosFil: Hoffman, Alexander F.. National Institutes of Health; Estados UnidosFil: Ladenheim, Bruce. National Institutes of Health; Estados UnidosFil: Cadet, Jean Lud. National Institutes of Health; Estados UnidosFil: Lupica, Carl R.. National Institutes of Health; Estados UnidosFil: Tagliaferro, Adriana. Universidad de Buenos Aires. Facultad de Medicina. Instituto de Biología Celular y Neurociencias; ArgentinaFil: Brusco, Herminia Alicia. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad de Buenos Aires. Facultad de Medicina. Instituto de Biología Celular y Neurociencias; ArgentinaFil: Bäckman, Cristina M.. National Institutes of Health; Estados Unido

Exposure to a novel environment does not enhance locomotor activity in <i>Pten</i> KO mice.

<p>Control (n = 11) and <i>Pten</i> KO (n = 15) animals were placed in activity chambers to measure the locomotor response to a novel environment. No significant differences were found in all parameters measured during a cummulative period of 30 minutes. Total distances traveled is shown as a representative measure. All data are mean±SEM. *p<0.05, Student's t-test.</p

The morphological preservation of the <i>Pten</i> deficient nigrostriatal system after 6OHDA treatment correlates with functional recovery after the lesion.

<p>To determine the functionality of the nigrostriatal system after the lesion, we examined methamphetamine-induced rotations. Methamphetamine treatment increases the extracellular availability of endogenous DA in the striatum, and it causes animals with a partial DA depletion to rotate against the lesion side (ipsilateral), due to the imbalance in DA release between the striatae. Strong ipsilateral rotational behavior was observed in control animals (n = 8) after treatment with (+) methamphetamine HCl (2.5 mg/kg) at 14 and 28 days after the lesion. Ipsilateral rotational behavior was significantly reduced in <i>Pten</i> KO animals (n = 10). All data are mean±SEM. *p<0.05, Student's t-test.</p

The lack of PTEN in DA neurons results in a significant enlargement of the ventral midbrain area, attributable to an increase in the total number of DA neurons that are larger in size, and display more dendritic extensions.

<p>(A) The volume of the substantia nigra compacta (SNc), ventral tegmental area (VTA) and substantia nigra reticulata (SNr) was significantly larger in <i>Pten</i> KO animals (n = 4), as compared to controls (n = 4). The volume of all measured areas was at least 40% larger in KO animals, with the largest increment in size seen in the SNr. (B) The increase in volume of the SNc and VTA from <i>Pten</i> KO animals was accompanied by a significant increase in the number of TH positive neurons (B) and cell size (C) in both regions. (B) The mean number of TH positive profiles in <i>Pten</i> KO animals was 9401±105.1 in the SNc, and 4775±106.3 in the VTA, a 43% and 26% increase, respectively, when compared to controls. (C) The area of TH positive neurons was 47% and 39% larger in the SNc and VTA of <i>Pten</i> KO animals as compared to controls. (D) The increase in the total volume covered by the SNr was attributable to an increase in the number and caliber of TH-positive fibers present in <i>Pten</i> KO animals in parallel with an increase in the number and size of TH positive cells in the SNc and VTA. All data are mean±SEM. *p<0.05, Student's t-test.</p

<i>Pten</i> ablation in DA cells affect the direct striatal output system by significantly increasing mRNA expression levels for DRD1 and pDyn in striatal medium spiny neurons.

<p>BDNF mRNA levels were significantly elevated in the ventral mesencephalon (A) Dopamie receptor D1 (DRD1) and D2 (DRD2) mRNA expression levels in the striatum of control (n = 9) and <i>Pten</i> KO animals (n = 13). DRD1 mRNA expression levels were significantly increased in KO animals as compared to controls. DRD2 mRNA levels were unchanged between the groups. (B) Changes in DRD1 expression levels were accompanied by an elevation in prodynorphin (pDyn) mRNA in <i>Pten</i> KO animals. Preproenkephalin (PPE) mRNA expression levels were unchanged. (C) BDNF mRNA levels were significantly increased in the ventral mesencephalon in KO animals. All data are mean±SEM. *p<0.05, Student's t-test.</p

Selective <i>Pten</i> deletion in DA cells results in unaltered extracellular dopamine dynamics in spite of a significant increase in tissue catecholamine content.

<p>(A) <i>Pten</i> KO mice showed no differences in basal extracellular dopamine as estimated by the x-intercept in the no net flux analysis. DA clearance was also unaltered as indicated by no change in the extraction fraction (slope of the no net flux regression line). (C) Depolarization evoked dopamine release was unaltered in <i>Pten</i> KO animals. The total tissue content of dopamine (DA) was increased in the dorsal striatum (B) and the midbrain (D) of <i>Pten</i> KO mice. Levels for the dopamine metablites DOPAC (dihydroxyphenylacetic acid), and HVA (homovanillic acid) were significantly inceased in the striatum and midbrain respectively. 3MT (3-methoxytyramine) levels remained unchanged as compared to controls. All data are mean±SEM. *p<0.05, Student's t-test.</p

<i>Pten</i> ablation shows neuroprotective effects in the intrastriatal 6OHDA mouse model.

<p>Animals were treated with a unilateral 6OHDA injection in the striatum, and perfused 4 weeks after the injection to analyze the effect of <i>Pten</i> ablation on the magnitude of the lesion. (A) The SNc from control animals (n = 4) was markedly affected by the lesion as reflected by the significant loss of TH positive profiles in the ipsilateral side to the lesion as compared to the contralateral side. The dopaminergic neurons in <i>Pten</i> KO (n = 6) animals were largely spared after the lesion when compared to control animals. (B) The neuroprotective effects related to <i>Pten</i> ablation were observed at the level of the striatal axonal projections as well. Depletion of TH positive terminals in the striatum was reduced across the striatum in control animals, while axon terminal degeneration was much lower in <i>Pten</i> KO animals. (C) Survival rates for TH-positive neurons were quantified by stereologic counts. The number of surviving neurons (presented as percent surviving neurons when compared to the side contralateral to the lesion) was significantly increased in <i>Pten</i> KO animals (n = 6) when compared to controls (n = 4). (D) Optical density measurement in the injected striatum revealed a nearly complete preservation of TH terminals in <i>Pten</i> KO animals. TH staining density was significantly lower in control 6OHDA-treated animals with the biggest difference observed in the medial portion of the striatum, in accordance with sterotaxic coordinated used for the lesion. (A) Scale bar, 250 µm and (B) 500 µm. All data are mean±SEM. *p<0.05, Student's t-test.</p