On the altered expression of tyrosine hydroxylase and calbindin-D 28kD immunoreactivities and viability of neurons in the ventral tegmental area of Tsai following injections of 6-hydroxydopamine in the medial forebrain bundle in the rat

Development of a unilaterally-lesioned 6-OHDA mouse model of Parkinson's disease

The unilaterally lesioned 6-hyroxydopamine (6-OHDA)-lesioned rat model of Parkinson's disease (PD) has proved to be invaluable in advancing our understanding of the mechanisms underlying parkinsonian symptoms, since it recapitulates the changes in basal ganglia circuitry and pharmacology observed in parkinsonian patients(1-4). However, the precise cellular and molecular changes occurring at cortico-striatal synapses of the output pathways within the striatum, which is the major input region of the basal ganglia remain elusive, and this is believed to be site where pathological abnormalities underlying parkinsonian symptoms arise(3,5). In PD, understanding the mechanisms underlying changes in basal ganglia circuitry following degeneration of the nigro-striatal pathway has been greatly advanced by the development of bacterial artificial chromosome (BAC) mice over-expressing green fluorescent proteins driven by promoters specific for the two striatal output pathways (direct pathway: eGFP-D1; indirect pathway: eGFP-D2 and eGFP-A2a)(8), allowing them to be studied in isolation. For example, recent studies have suggested that there are pathological changes in synaptic plasticity in parkinsonian mice(9,10). However, these studies utilised juvenile mice and acute models of parkinsonism. It is unclear whether the changes described in adult rats with stable 6-OHDA lesions also occur in these models. Other groups have attempted to generate a stable unilaterally-lesioned 6-OHDA adult mouse model of PD by lesioning the medial forebrain bundle (MFB), unfortunately, the mortality rate in this study was extremely high, with only 14% surviving the surgery for 21 days or longer(11). More recent studies have generated intra-nigral lesions with both a low mortality rate >80% loss of dopaminergic neurons, however expression of L-DOPA induced dyskinesia(11,12,13,14) was variable in these studies. Another well established mouse model of PD is the MPTP-lesioned mouse(15). Whilst this model has proven useful in the assessment of potential neuroprotective agents(16), it is less suitable for understanding mechanisms underlying symptoms of PD, as this model often fails to induce motor deficits, and shows a wide variability in the extent of lesion(17, 18). Here we have developed a stable unilateral 6-OHDA-lesioned mouse model of PD by direct administration of 6-OHDA into the MFB, which consistently causes >95% loss of striatal dopamine (as measured by HPLC), as well as producing the behavioural imbalances observed in the well characterised unilateral 6-OHDA-lesioned rat model of PD. This newly developed mouse model of PD will prove a valuable tool in understanding the mechanisms underlying generation of parkinsonian symptoms.

Generation of a model of L-DOPA-induced dyskinesia in two different mouse strains

Prolonged use of the dopamine precursor L-DOPA for the treatment of Parkinson's disease commonly results in abnormal involuntary movements, which are termed L-DOPA-induced dyskinesia (LID). Over-activity at corticostriatal synapses onto neurons of the direct and indirect striatal output pathways has been implicated in the development of dyskinesia, but it has proved difficult to investigate the pathways separately due to their morphological similarities. The recent development of bacterial artificial chromosome mice that express green fluorescent protein in either the direct or indirect pathway allows visual identification of the output neurons in each pathway. Here we describe the use of two different strains of these transgenic mice (pure FVB and FVB crossed with C57BL6) in the development of mouse models of L-DOPA-induced dyskinesia. This model will allow the direct and indirect pathways to be studied individually to delineate the cellular and molecular mechanisms underlying dyskinesias. These studies demonstrate that mouse strain impacts on behavioural responses and L-DOPA sensitivity. Therefore, when generating mouse models of LID, strain must be taken into consideration when choosing the L-DOPA dosing regimen.

Survival and early functional integration of dopaminergic progenitor cells following transplantation in a rat model of Parkinson's disease

Dopaminergic (DA) grafts in rat models of Parkinson's disease (PD) have previously been derived from embryonic day (E) 14 grafts. Because there is an increasing interest in the restorative capacity of DA stem and progenitor cells, in the present study we examined the survival and early and late functional behavioral effects of DA progenitor cells derived from E12, E13, E14, and E15 grafts transplanted into rats with unilateral 6-hydroxydopamin lesions. DA transplant-induced functional recovery was already observed in postural balancing reactions after 10 days and in stepping behavior after 13 days, that is, in spontaneous complex behaviors, and later, after 16 days, in the amphetamine-induced rotation test. Three distinct patterns of functional recovery could be observed at 6-9 weeks posttransplantation. First, behavioral improvements in drug-induced rotational asymmetry, stepping, and skilled forelimb behavior were directly related to DA neuron survival and TH-positive fiber reinnervation. Second, recovery in postural balancing reactions was closely related to a specific developmental time window of donor age, for example, only seen in E13 and E14 grafts. Finally, no functional graft effects were seen in the table lift test. Interestingly, DA neuron graft survival, TH-positive fiber outgrowth, and graft volume were significantly influenced by the developmental time window in which the DA progenitor cells were dissected from the ventral mesencephalon, that is, from E12, E13, E14, or E15 rat embryos. These data highlight the complexity of graft-host interactions and provide novel insights into the dynamics of DA progenitor graft-mediated functional recovery in animal models of Parkinson's disease.

Effects of calpain inhibition on dopaminergic markers and motor function following intrastriatal 6-hydroxydopamine administration in rats

The neurotoxin 6-hydroxydopamine has been widely used to model aspects of Parkinson's disease in rodents, but the mechanisms underlying toxin-induced dopaminergic degeneration and functional impairment have not been fully elucidated. The main aim of the present study was to assess a possible role for calpains in neurochemical and behavioral deficits following unilateral infusion of intrastriatal 6-hydroxydopamine in adult rats. Toxin administration produced a profound dopaminergic denervation, as indicated by a 90-95% reduction in dopamine transporter radiolabeling measured in the caudate-putamen at 2 weeks post-lesion. Treatment with 6-hydroxydopamine also resulted in calpain activation in both caudate-putamen and substantia nigra, as measured by the appearance of calpain-specific spectrin breakdown products. Calpain activation peaked at 24 h after 6-hydroxydopamine infusion and remained elevated at later time points. In contrast, caspase-3-mediated spectrin cleavage subsided within 48 h in both brain areas. In a subsequent experiment, calpain inhibition was achieved by intrastriatal infusion of an adenovirus expressing the endogenous calpain inhibitor, calpastatin. Calpastatin delivery abolished the lesion-induced calpain-mediated spectrin cleavage and alleviated forelimb asymmetries resulting from unilateral intrastriatal 6-hydroxydopamine. Unexpectedly, dopamine transporter and tyrosine hydroxylase labeling revealed significant neuroprotection, not in the nigrostriatal pathway but rather in the ventral tegmental area. These findings support a role for calpain activation in 6-hydroxydopamine-induced degeneration of dopaminergic neurons. However, after near-total dopaminergic depletion, the primary benefit of calpain inhibition may not occur within the nigrostriatal dopaminergic pathway itself.

Acetaminophen attenuates dopamine neuron degeneration in animal models of Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disorder with approximately 2% of people over age 65 suffering from this disease. Risk factors for PD involve interplay between still poorly defined genetic and non-genetic contributors, but appear to converge upon cellular pathways that mediate protein misfolding and oxidative stress that lead to dopaminergic neuron loss. The identification of either new or repurposed drugs that exhibit benefit in slowing the age-dependent neuronal damage that occurs in PD is a significant goal of much ongoing research. We have exploited the nematode Caenorhabditis elegans as a model system by which the neuroprotective capacity of acetaminophen could be rapidly evaluated for efficacy in attenuating dopamine (DA) neurodegeneration. Using three independent and established neurodegenerative models in C. elegans, we assayed for acetaminophen-dependent rescue in response to: (1) over-expression of the PD-associated protein, alpha-synuclein; (2) acute exposure to 6-hydroxydopamine (6-OHDA); (3) excess intracellular DA production due to over-expression of the DA biosynthetic enzyme, tyrosine hydroxylase (TH). These data suggest that acetaminophen significantly protected C. elegans DA neurons from stressors related to oxidative damage, but not protein misfolding. Taken together, these studies imply an activity for acetaminophen in the attenuation of DA neuron loss that, following essential corroborative analyses in mammalian systems, may represent a potential benefit for PD.

Molecular and cellular alterations in the Pitx3-deficient midbrain dopaminergic system

Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized by loss of midbrain dopaminergic (mDA) neurons in the substantia nigra compacta (SNc). In order to provide insights into adaptive mechanisms of the mDA system in pathology, specific molecular and cellular parameters of the mDA system were studied in Pitx3-deficient Aphakia (ak) mice, which suffer from severe developmental failure of SNc mDA neurons. Here, we demonstrate differential changes in striatal gene expression, reflecting the specific neuronal loss in these mice. In addition, the neuronal activity of remaining mDA neurons in the ventral tegmental area (VTA) was significantly increased in ak mice. In conclusion, ak mice display specific molecular and cellular alterations in the mDA system that provide new insights in compensatory mechanisms present in mDA-associated disorders such as PD.

Vulnerabilities of ventral mesencephalic neurons projecting to the nucleus accumbens following infusions of 6-hydroxydopamine into the medial forebrain bundle in the rat

The terminal arbors of dopaminergic projections in the nucleus accumbens (Acb) core degenerate more rapidly, completely and permanently in a variety of neurotoxic circumstances than do those in the medial shell. It is unknown if this always reflects purely losses of the distal parts of axons from the core (as proposed in methamphetamine intoxication), or whether, in some circumstances, the disproportionate loss of core axons may also stem from an intrinsic vulnerability to degeneration of core-projecting neuronal perikarya. Experiments described here addressed this issue in the following manner. Three days after Fluoro-Gold (FG), a retrogradely transported tracer, had been iontophoresed selectively into the core or medial shell of male Sprague-Dawley rats, each received an infusion of saline vehicle containing or lacking 6-hydroxydopamine (6-OHDA) in the ipsilateral medial forebrain bundle (MFB). Twenty-one days later the brains were processed to exhibit ventral mesencephalic neurons containing FG. Application of an unbiased sampling method revealed substantially greater losses of FG labeled neurons relative to controls in rats that had received 6-OHDA lesions and deposition of FG in the Acb core as compared to the medial shell. Of the few core-projecting neurons that remained in the ventral mesencephalon after these lesions, 54% did not co-localize tyrosine hydroxylase immunoreactivity (TH-ir) and, thus, were not expected to degenerate. The capacity to selectively remove core-projecting dopaminergic neurons may be useful in the determination of molecular correlates of vulnerability and resistance to neurotoxicity and to possibly test the role of the core in reinforcement paradigms.

3,4-Dihydroxyphenylacetaldehyde is the toxic dopamine metabolite in vivo: implications for Parkinson's disease pathogenesis

In Parkinson's disease (PD), there is a highly selective loss of dopamine (DA) neurons in the substantia nigra (SN) greater than in the ventral tegmental area (VTA). The simplest explanation for selective DA neuron loss in PD is that DA is toxic and, because only DA neurons contain significant amounts of DA, this highly localized synthesis of DOPAL accounts for selective vulnerability of DA neurons. However, the large concentrations of DA required to produce in vivo toxicity cast doubt on its role in PD pathogenesis. Alpha-synuclein (alpha-syn) is the major component of the Lewy body, the pathological marker of PD, and is genetically linked to the disease. Recent studies indicate that alpha-syn neurotoxicity is mediated by a free radical generating metabolite of DA. Here we test the hypothesis that 3,4-dihydroxyphenylacetaldehyde (DOPAL), the monamine oxidase metabolite of DA, mediates DA toxicity in vivo. We injected DOPAL, DA and its oxidative, reduced and methylated metabolites into rat SN and VTA. Five days post-surgery, the injection sites were evaluated in Nissl preparations and with tyrosine hydroxylase (for DA neurons), neuronal nuclear antigen (for neurons) and glial fibrillary acidic protein (for astrocytes) immunoreactivities. Lesion size in SN vs. VTA was compared using morphometry. DOPAL at concentrations as low as 100 ng was toxic to DA SN neurons>DA VTA neurons>glia. Neither DA nor its other metabolites showed evidence of neurotoxicity at fivefold higher doses. However, 20 microg of DA produced lesions in the SN and VTA. We conclude that DOPAL is the toxic DA metabolite in vivo. Implications for a unified hypothesis for PD pathogenesis are discussed.

Effect of 6-hydroxydopamine on striatal GDNF and nigral GFRα1 and RET mRNAs in the adult rat

Exogenous GDNF as well as vectors containing the gene for this trophic factor has been shown to be neuroprotective in animal models of Parkinson's disease. We therefore investigated whether changes in striatal GDNF protein and nigral mRNA levels of its co-receptors GFRalpha1 and RET occur in response to lesions of dopamine (DA) neurons and examined the temporal profile of these changes as they relate to the loss of dopaminergic markers. Rats were lesioned with 6-hydroxydopamine and sacrificed 3 h to 60 days post-infusion. DA tissue levels in the striatum and tyrosine hydroxylase immunoreactivity in the substantia nigra (SN) and ventral tegmental area (VTA) were used to determine the size of the lesions. GDNF protein was measured in the striatum using radioimmunocytochemistry. In situ hybridization was used to determine alterations in the mRNAs of RET and GFRalpha1 in the SN and VTA. We observed no persistent changes in GDNF protein in the striatum in response to 6-hydroxydopamine over the 60-day observation period, suggesting that compensatory changes in this trophic factor do not occur in response to injury. Dramatic decreases in RET and GFRalpha1 were observed in both SN and VTA that were generally correlated with the loss of TH protein and striatal DA content, strongly suggesting that these receptors are located on DA neurons and that the protective effect of GDNF reflects a direct action of the trophic factor on these neurons.

Susceptibility of ascending dopamine projections to 6-hydroxydopamine in rats: effect of hypothermia

The aims of this study were to determine (1) whether mesolimbic and nigrostriatal DA cell bodies degenerate to different extents after 6-hydroxydopamine (6-OHDA) is administered into their respective terminal fields and (2) whether hypothermia, associated with sodium pentobarbital anesthesia, protects DA neurons from the toxic effects of 6-OHDA. To address these questions, 6-OHDA or vehicle was infused into either the ventral or dorsal striatum or into the medial forebrain bundle, under conditions of brain normothermia or hypothermia. Two weeks post-surgery, tyrosine hydroxylase-positive cell bodies were counted in the ventral tegmental area (VTA) and substantia nigra. In addition, autoradiographic labeling of tyrosine hydroxylase protein and dopamine transporter was quantified in dopamine terminal fields and cell body areas. Overall, DA cell bodies in the VTA were substantially less susceptible than those in the substantia nigra to depletion of dopaminergic markers. Hypothermia provided two types of neuroprotection. The first occurred when 6-OHDA was administered into the dorsal striatum, and was associated with a 30-50% increase in residual dopaminergic markers in the lateral portion of the VTA. The second neuroprotective effect of hypothermia occurred when 6-OHDA was given into the medial forebrain bundle. This was associated with a 200-300% increase in residual dopaminergic markers in the mesolimbic and nigrostriatal terminal fields; no significant protection occurred in the cell body regions.Collectively, these findings show that (1) the dopaminergic somata in the substantia nigra are more susceptible than those in the VTA to 6-OHDA-induced denervation, and (2) hypothermia can provide anatomically selective neuroprotection within the substantia nigra-VTA cell population. The continued survival of mesolimbic dopamine cell bodies after a 6-OHDA lesion may have functional implications relating to drugs of abuse, as somatodendritic release of dopamine in the VTA has been shown to play a role in the effectiveness of cocaine reward.

Mitochondrial complex inhibitors preferentially damage substantia nigra dopamine neurons in rat brain slices

Using a rat brain slice preparation, we investigated the role of energy impairment on the selective loss of dopamine neurons in the substantia nigra (SN). Brain slices (400 microm) were incubated at 35 degrees C for 2 h in the presence or absence of mitochondrial complex inhibitors, rotenone, MPP+, 3-nitropropionic acid, and antimycin A. Slices were also incubated in rotenone with excitatory amino acid (EAA) receptor antagonists, MK-801 and CNQX, to determine whether rotenone-induced damage was mediated by EAAs. The slices were then fixed, recut into 30-microm sections, and immunolabeled for tyrosine hydroxylase (TH) to identify catecholamine neurons and to quantify loss of TH-labeled dendrites after treatment. Quantitative comparison was made between SN dopamine neurons, in which rotenone-induced dendrite loss was severe, and hypothalamic A11 dopamine neurons, which were spared. Adjacent sections that were immunolabeled for calbindin or stained with cresyl violet also revealed a striking dendritic degeneration of SN neurons in rotenone-exposed slices, whereas noncatecholamine neurons, such as those in the perifornical nucleus (PeF), were more resistant. Preferential damage to SN dopamine neurons was also evident with other mitochondrial complex inhibitors, MPP+ and antimycin A. EAA receptor antagonists provided partial protection to SN neurons in slices incubated with rotenone (3 microM). The particular vulnerability of SN dopamine neurons in the slice is consistent with the vulnerability of SN in Parkinson's disease. The selective effect of mitochondrial complex inhibition in SN dopamine neurons implies a fundamental deficit in the capacity of these neurons to defend against toxic insult.

I(h) channels contribute to the different functional properties of identified dopaminergic subpopulations in the midbrain

Dopaminergic (DA) midbrain neurons in the substantia nigra (SN) and ventral tegmental area (VTA) are involved in various brain functions such as voluntary movement and reward and are targets in disorders such as Parkinson's disease and schizophrenia. To study the functional properties of identified DA neurons in mouse midbrain slices, we combined patch-clamp recordings with either neurobiotin cell-filling and triple labeling confocal immunohistochemistry, or single-cell RT-PCR. We discriminated four DA subpopulations based on anatomical and neurochemical differences: two calbindin D28-k (CB)-expressing DA populations in the substantia nigra (SN/CB+) or ventral tegmental area (VTA/CB+), and respectively, two calbindin D28-k negative DA populations (SN/CB-, VTA/CB-). VTA/CB+ DA neurons displayed significantly faster pacemaker frequencies with smaller afterhyperpolarizations compared with other DA neurons. In contrast, all four DA populations possessed significant differences in I(h) channel densities and I(h) channel-mediated functional properties like sag amplitudes and rebound delays in the following order: SN/CB- --> VTA/CB- --> SN/CB+ --> VTA/CB+. Single-cell RT-multiplex PCR experiments demonstrated that differential calbindin but not calretinin expression is associated with differential I(h) channel densities. Only in SN/CB- DA neurons, however, I(h) channels were actively involved in pacemaker frequency control. In conclusion, diversity within the DA system is not restricted to distinct axonal projections and differences in synaptic connectivity, but also involves differences in postsynaptic conductances between neurochemically and topographically distinct DA neurons.