Wayne A. Cass

Inflammation induces mitochondrial dysfunction and dopaminergic neurodegeneration in the nigrostriatal system

Evidence suggests that chronic inflammation, mitochondrial dysfunction, and oxidative stress play significant and perhaps synergistic roles in Parkinson’s disease (PD), where the primary pathology is significant loss of the dopaminergic neurons in the substantia nigra. The use of anti‐inflammatory drugs for PD treatment has been proposed, and inhibition of cyclo‐oxygenase‐2 (COX‐2) or activation of peroxisome proliferator‐activated receptor gamma (PPAR‐γ) yields neuroprotection in MPTP‐induced PD. Lipopolysaccharide (LPS) induces inflammation‐driven dopaminergic neurodegeneration. We tested the hypothesis that celecoxib (Celebrex, COX‐2 inhibitor) or pioglitazone (Actos, PPAR‐γ agonist) will reduce the LPS‐induced inflammatory response, spare mitochondrial bioenergetics, and improve nigral dopaminergic neuronal survival. Rats were treated with vehicle, celecoxib, or pioglitazone and were intrastriatally injected with LPS. Inflammation, mitochondrial dysfunction, oxidative stress, decreased dopamine, and nigral dopaminergic neuronal loss were observed post‐LPS. Celecoxib and pioglitazone provided neuroprotective properties by decreasing inflammation and restoring mitochondrial function. Pioglitazone also attenuated oxidative stress and partially restored striatal dopamine as well as demonstrated dopaminergic neuroprotection and reduced nigral microglial activation. In summary, intrastriatal LPS served as a model for inflammation‐induced dopaminergic neurodegeneration, anti‐inflammatory drugs provided protective properties, and pioglitazone or celecoxib may have therapeutic potential for the treatment of neuro‐inflammation and PD.

Molecular basis for interactions of HIV and drugs of abuse.

Summary: In certain populations around the world, the HIV pandemic is being driven by drug‐abusing populations. Mounting evidence suggests that these patient populations have accelerated and more severe neurocognitive dysfunction compared with non‐drug‐abusing HIV‐infected populations. Because most drugs of abuse are central nervous system stimulants, it stands to reason that these drugs may synergize with neurotoxic substances released during the course of HIV infection. Clinical and laboratory evidence suggests that the dopaminergic systems are most vulnerable to such combined neurotoxicity. Identifying common mechanisms of neuronal injury is critical to developing therapeutic strategies for drug‐abusing HIV‐infected populations. This article reviews 1) the current evidence for neurodegeneration in the setting of combined HIV infection and use of methamphetamine, cocaine, heroin or alcohol; 2) the proposed underlying mechanisms involved in this combined neurotoxicity; and 3) future directions for research. This article also suggests therapeutic approaches based on our current understanding of the neuropathogenesis of dementia due to HIV infection and drugs of abuse.

Direct in vivo evidence that D2 dopamine receptors can modulate dopamine uptake

In vivo electrochemistry was used to determine the effects of locally applied raclopride (a D2 receptor antagonist) and SCH-23390 (a D1 receptor antagonist) on the clearance of locally applied dopamine in the striatum, nucleus accumbens, and medial prefrontal cortex of rats. Chronoamperometric recordings were continuously made at 5 Hz using Nafion-coated, single carbon fiber electrodes. When a calibrated amount of dopamine was pressure ejected at 5-min intervals from a micropipette adjacent (280-310 microns) to the electrode, transient and reproducible dopamine signals were detected in all three regions. Local application of raclopride from a second micropipette, prior to pressure ejection of dopamine, increased the amplitude and time course of the dopamine signals, indicating significant inhibition of the dopamine transporter. In contrast, local application of SCH-23390 or saline had no effect on the dopamine signals. These data indicate that D2, but not D1, dopamine receptors can modulate the activity of the dopamine transporter.

In Vivo Assessment of Dopamine Uptake in Rat Medial Prefrontal Cortex: Comparison with Dorsal Striatum and Nucleus Accumbens

Abstract: In vivo electrochemistry was used to characterize dopamine clearance in the medial prefrontal cortex and to compare it with clearance in the dorsal striatum and nucleus accumbens. When calibrated amounts of dopamine were pressure‐ejected into the cortex from micropipettes adjacent to the recording electrodes, transient and reproducible dopamine signals were detected. The local application of the selective uptake inhibitors GBR‐12909, desipramine, and fluoxetine before the application of dopamine indicated that at the lower recording depths examined (2.5–5.0 mm below the brain surface), locally applied dopamine was cleared from the extracellular space primarily by the dopamine transporter. The norepinephrine transporter played a greater role at the more superficial recording sites (0.5–2.25 mm below the brain surface). To compare clearance of dopamine in the medial prefrontal cortex (deeper sites only), striatum, and nucleus accumbens, varying amounts of dopamine were locally applied in all three regions of individual animals. The signals recorded from the cortex were of greater amplitude and longer time course than those recorded from the striatum or accumbens (per picomole of dopamine applied), indicating less efficient dopamine uptake in the medial prefrontal cortex. The fewer number of transporters in the medial prefrontal cortex may be responsible, in part, for this difference, although other factors may also be involved. These results are consistent with the hypothesis that regulation of dopaminergic function is unique in the medial prefrontal cortex.

Trichloroethylene: Parkinsonism and complex 1 mitochondrial neurotoxicity

To analyze a cluster of 30 industrial coworkers with Parkinsons disease and parkinsonism subjected to long‐term (8–33 years) chronic exposure to trichloroethylene.

GDNF Protection against 6-OHDA: Time Dependence and Requirement for Protein Synthesis

Glial cell line-derived neurotrophic factor (GDNF) injected intranigrally protects midbrain dopamine neurons against 6-hydroxydopamine (6-OHDA) toxicity. The timing between GDNF administration and exposure to 6-OHDA is critical in achieving optimal protection. When injected 6 hr before an intranigral injection of 6-OHDA, GDNF provides complete protection as measured by the number of surviving neurons in the substantia nigra of adult rats. The surviving neuronal population decreases by ∼50% with 12 and 24 hr separating GDNF and 6-OHDA administrations. In controls with 6-OHDA lesions, there is <10% survival of nigral dopamine neurons. No significant increase in survival is seen with either concurrent injections of GDNF and 6-OHDA or 1 hr GDNF pretreatment. Based on HPLC measurements, striatal and midbrain dopamine levels are at least twofold higher on the lesioned side in animals receiving GDNF 6 hr before a 6-OHDA lesion compared with vehicle recipients. Protein synthesis is necessary for GDNF-induced neuroprotective effects because cycloheximide pretreatment that inhibits protein synthesis also blocks neuroprotection.

Glial Cell Line-Derived Neurotrophic Factor Increases Stimulus-Evoked Dopamine Release and Motor Speed in Aged Rhesus Monkeys

Changes in the functional dynamics of dopamine release and regulation in the basal ganglia have been posited to contribute to age-related slowing of motor functions. Here, we report the effects of glial cell line-derived neurotrophic factor (GDNF) on the stimulus-evoked release of dopamine and motor speed in aged monkeys (21–27 years of age; n = 10). Although no changes were observed in the vehicle controls (n = 5), chronic infusions of 7.5 μg of GDNF per day for 2 months into the right lateral ventricle initially increased hand movement speed up to 40% on an automated hand-reach task. These effects were maintained for at least 2 months after replacing GDNF with vehicle, and increased up to another 10% after the reinstatement of GDNF treatment for 1 month. In addition, upper-limb motor performance times of the aged GDNF-treated animals (n = 5) recorded at the end of the study were similar to those of five young adult monkeys (8–12 years of age). The stimulus-evoked release of dopamine was significantly increased, up to 130% in the right caudate nucleus and putamen and up to 116% in both the right and left substantia nigra of the aged GDNF recipients compared with vehicle controls. Also, basal extracellular levels of dopamine were bilaterally increased, up to 163% in the substantia nigra of the aged GDNF-treated animals. The data suggest that the effects of GDNF on the release of dopamine in the basal ganglia may be responsible for the improvements in motor functions and support the hypothesis that functional changes in dopamine release may contribute to motor dysfunctions characterizing senescence.

Increased Mitochondrial Calcium Sensitivity and Abnormal Expression of Innate Immunity Genes Precede Dopaminergic Defects in Pink1-Deficient Mice

Background PTEN-induced kinase 1 (PINK1) is linked to recessive Parkinsonism (EOPD). Pink1 deletion results in impaired dopamine (DA) release and decreased mitochondrial respiration in the striatum of mice. To reveal additional mechanisms of Pink1-related dopaminergic dysfunction, we studied Ca2+ vulnerability of purified brain mitochondria, DA levels and metabolism and whether signaling pathways implicated in Parkinsons disease (PD) display altered activity in the nigrostriatal system of Pink1−/− mice. Methods and Findings Purified brain mitochondria of Pink1−/− mice showed impaired Ca2+ storage capacity, resulting in increased Ca2+ induced mitochondrial permeability transition (mPT) that was rescued by cyclosporine A. A subpopulation of neurons in the substantia nigra of Pink1−/− mice accumulated phospho-c-Jun, showing that Jun N-terminal kinase (JNK) activity is increased. Pink1−/− mice 6 months and older displayed reduced DA levels associated with increased DA turnover. Moreover, Pink1−/− mice had increased levels of IL-1β, IL-12 and IL-10 in the striatum after peripheral challenge with lipopolysaccharide (LPS), and Pink1−/− embryonic fibroblasts showed decreased basal and inflammatory cytokine-induced nuclear factor kappa-β (NF-κB) activity. Quantitative transcriptional profiling in the striatum revealed that Pink1−/− mice differentially express genes that (i) are upregulated in animals with experimentally induced dopaminergic lesions, (ii) regulate innate immune responses and/or apoptosis and (iii) promote axonal regeneration and sprouting. Conclusions Increased mitochondrial Ca2+ sensitivity and JNK activity are early defects in Pink1−/− mice that precede reduced DA levels and abnormal DA homeostasis and may contribute to neuronal dysfunction in familial PD. Differential gene expression in the nigrostriatal system of Pink1−/− mice supports early dopaminergic dysfunction and shows that Pink1 deletion causes aberrant expression of genes that regulate innate immune responses. While some differentially expressed genes may mitigate neurodegeneration, increased LPS-induced brain cytokine expression and impaired cytokine-induced NF-κB activation may predispose neurons of Pink1−/− mice to inflammation and injury-induced cell death.

Human immunodeficiency virus‐1 Tat protein and methamphetamine interact synergistically to impair striatal dopaminergic function

The human immunodeficiency virus (HIV)‐1 transactivating protein Tat may be pathogenically relevant in HIV‐1‐induced neuronal injury. The abuse of methamphetamine (MA), which is associated with behaviors that may transmit HIV‐1, may damage dopaminergic afferents to the striatum. Since Tat and MA share common mechanisms of injury, we examined whether co‐exposure to these toxins would lead to enhanced dopaminergic toxicity. Animals were treated with either saline, a threshold dose of MA, a threshold concentration of Tat injected directly into the striatum, or striatal injections of Tat followed by exposure to MA. Threshold was defined as the highest concentration of toxin that would not result in a significant loss of striatal dopamine levels. One week later, MA‐treated animals demonstrated a 7% decline in striatal dopamine levels while Tat‐treated animals showed an 8% reduction. Exposure to both MA + Tat caused an almost 65% reduction in striatal dopamine. This same treatment caused a 56% reduction in the binding capacity to the dopamine transporter. Using human fetal neurons, enhanced toxicity was also observed when cells were exposed to both Tat and MA. Mitochondrial membrane potential was disrupted and could be prevented by treatment with antioxidants. This study demonstrates that the HIV‐1 ‘virotoxin’ Tat enhances MA‐induced striatal damage and suggests that HIV‐1‐infected individuals who abuse MA may be at increased risk of basal ganglia dysfunction.

Changes in somatodendritic but not terminal dopamine regulation in aged rhesus monkeys

For these studies, young (8–9 years), middle‐aged (14–17 years) and aged (23–28 years) rhesus monkeys were used as a model of normal aging in humans to investigate changes in dopamine (DA)‐containing neurons in senescence. Aged monkeys exhibited significant age‐related motoric declines as compared to the young animals. In vivo microdialysis studies showed that basal levels of the DA metabolites, homovanillic acid (HVA) and 3,4‐dihydroxyphenylacetic acid (DOPAC) were diminished by 44% and 79%, respectively, in␣the substantia nigra (SN) of aged monkeys. In addition, d‐amphetamine‐evoked overflow of DA in the SN was diminished by 30% in the middle‐aged animals and 67% in the aged monkeys. Post‐mortem measures of DA and DA metabolites showed significant decreases in DA (20%), DOPAC (47%) and HVA (22%) levels in the putamen and a 25% decline in HVA tissue levels in the SN of the aged monkeys as compared to the young animals. Unbiased stereological cell counting of tyrosine hydroxylase (TH)‐immunoreactive neurons in the SN showed a small (15–20%) but significant age‐related decline in TH‐positive neurons. In addition, there was a small (15–20%) but significant decline in TH‐positive fiber density and TH‐positive cell size. In comparison to the massive loss of DA neurons responsible for the movement dysfunctions seen in Parkinsons disease, pronounced functional changes in DA release in the SN and putamen may significantly contribute to the motoric dysfunctions characterizing normal aging in rhesus monkeys.