Astrocytes and retrograde degeneration of nigrostriatal dopaminergic neurons in Parkinson’s disease: removing axonal debris

Objective The dopaminergic nigrostriatal neurons (DA cells) in healthy people present a slow degeneration with aging, which produces cellular debris throughout life. About 2%–5% of people present rapid cell degeneration of more than 50% of DA cells, which produces Parkinson’s disease (PD). Neuroinflammation accelerates the cell degeneration and may be critical for the transition between the slow physiological and the rapid pathological degeneration of DA cells, particularly when it activates microglial cells of the medial forebrain bundle near dopaminergic axons. As synaptic debris produced by DA cell degeneration may trigger the parkinsonian neuroinflammation, this study investigated the removal of axonal debris produced by retrograde degeneration of DA cells, paying particular attention to the relative roles of astrocytes and microglia. Methods Rats and mice were injected in the lateral ventricles with 6-hydroxydopamine, inducing a degeneration of dopaminergic synapses in the striatum which was not accompanied by non-selective tissue damage, microgliosis or neuroinflammation. The possible retrograde degeneration of dopaminergic axons, and the production and metabolization of DA-cell debris were studied with immunohistochemical methods and analyzed in confocal and electron microscopy images. Results The selective degeneration of dopaminergic synapses in the striatum was followed by a retrograde degeneration of dopaminergic axons whose debris was found within spheroids of the medial forebrain bundle. These spheroids retained mitochondria and most (e.g., tyrosine hydroxylase, the dopamine transporter protein, and amyloid precursor protein) but not all (e.g., α-synuclein) proteins of the degenerating dopaminergic axons. Spheroids showed initial (autophagosomes) but not late (lysosomes) components of autophagy (incomplete autophagy). These spheroids were penetrated by astrocytic processes of the medial forebrain bundle, which provided the lysosomes needed to continue the degradation of dopaminergic debris. Finally, dopaminergic proteins were observed in the cell somata of astrocytes. No microgliosis or microglial phagocytosis of debris was observed in the medial forebrain bundle during the retrograde degeneration of dopaminergic axons. Conclusions The present data suggest a physiological role of astrocytic phagocytosis of axonal debris for the medial forebrain bundle astrocytes, which may prevent the activation of microglia and the spread of retrograde axonal degeneration in PD.

Antinociceptive effect of melatonin in the animal model of Parkinson’s Disease

Several animal experimental and clinical studies have shown the effectiveness of melatonin in the treatment of some symptoms of Parkinson's disease (PD). However, the antinociceptive effect of melatonin against pain associated to PD has not been fully investigated. Thus, the present study investigated the possible antiallodynic and antinociceptive effects of acute and chronic melatonin treatments in Parkinsonian model of rats. This model was created by unilateral injection of 6-hydroxydopamine (6-OHDA) into the left medial forebrain bundle (MFB). The electronic von Frey test was used to analyze the antiallodynic effect of melatonin on this PD animal model. In addition, c-Fos immunostaining was also used as a marker of nociception to evaluate the neuronal activity related to the nociception processing. The results showed that unilateral injection of 6-OHDA induced a significant decrease in paw withdrawal threshold in both ipsilateral and contralateral paws, which indicate mechanical allodynia induction. This allodynia was transitorily reversed by apomorphine as a dopamine agonist. Melatonin treatment significantly increased threshold of allodynia. Melatonin administration of both acutely or chronically significantly downregulated the c-Fos expression of neurons in 6-OHDA treated animals. In conclusion, 6-OHDA treatment can induces a bilateral mechanical hypernociception in rats while melatonin treatment produces profound antinociceptive effect. This finding paves the way to use melatonin as an antinociceptive agent for PD clinically.

Acupuncture Modulates Intracranial Self-Stimulation of the Medial Forebrain Bundle in Rats

Acupuncture affects the central nervous system via the regulation of neurotransmitter transmission. We previously showed that Shemen (HT7) acupoint stimulation decreased cocaine-induced dopamine release in the nucleus accumbens. Here, we used the intracranial self-stimulation (ICSS) paradigm to evaluate whether HT stimulation regulates the brain reward function of rats. We found that HT stimulation triggered a rightward shift of the frequency–rate curve and elevated the ICSS thresholds. However, HT7 stimulation did not affect the threshold-lowering effects produced by cocaine. These results indicate that HT7 points only effectively regulates the ICSS thresholds of the medial forebrain bundle in drug-naïve rats.

Riluzole Administration to Rats with Levodopa-Induced Dyskinesia Leads to Loss of DNA Methylation in Neuronal Genes

Dyskinesias are characterized by abnormal repetitive involuntary movements due to dysfunctional neuronal activity. Although levodopa-induced dyskinesia, characterized by tic-like abnormal involuntary movements, has no clinical treatment for Parkinson’s disease patients, animal studies indicate that Riluzole, which interferes with glutamatergic neurotransmission, can improve the phenotype. The rat model of Levodopa-Induced Dyskinesia is a unilateral lesion with 6-hydroxydopamine in the medial forebrain bundle, followed by the repeated administration of levodopa. The molecular pathomechanism of Levodopa-Induced Dyskinesia is still not deciphered; however, the implication of epigenetic mechanisms was suggested. In this study, we investigated the striatum for DNA methylation alterations under chronic levodopa treatment with or without co-treatment with Riluzole. Our data show that the lesioned and contralateral striata have nearly identical DNA methylation profiles. Chronic levodopa and levodopa + Riluzole treatments led to DNA methylation loss, particularly outside of promoters, in gene bodies and CpG poor regions. We observed that several genes involved in the Levodopa-Induced Dyskinesia underwent methylation changes. Furthermore, the Riluzole co-treatment, which improved the phenotype, pinpointed specific methylation targets, with a more than 20% methylation difference relative to levodopa treatment alone. These findings indicate potential new druggable targets for Levodopa-Induced Dyskinesia.

Green coffee extract attenuates Parkinson’s-related behaviors in animal models

Epidemiological studies have shown an inverse association between coffee consumption and the development of Parkinson’s disease (PD). The present investigation aimed to evaluate the effects of caffeine and green (non-roasted) coffee extract in experimental models of PD. The effects of the oral treatment with green coffee extracts (CE, Coffea arabica 100 or 400 mg/kg) and caffeine (31.2 mg/kg) were evaluated on catalepsy induced by haloperidol in mice, and unilateral 6-OHDA lesion of medial forebrain bundle (MFB) or striatum in rats. Also, the in vitro antioxidant activity and the monoamine levels in the striatum were investigated. CE presented a mild antioxidant activity in vitro and its administration decreased the catalepsy index. CE at the dose of 400 mg/kg induced ipsilateral rotations 14 days after the lesion; however, chronic 30-day CE and caffeine treatments did not interfere with the animals’ rotation after apomorphine or methamphetamine challenges in animals with MFB lesion, nor on monoamines levels. Furthermore, CE and caffeine were effective in inhibiting the asymmetry between ipsilateral and contralateral rotations induced by methamphetamine and apomorphine in animals with lesion in the striatum but did not avoid the monoamines depletion. These results suggest a pro-dopaminergic action of CE, although its mechanism remains unclear.