Methylphenidate Exposure Induces Dopamine Neuron Loss and Activation of Microglia in the Basal Ganglia of Mice

BackgroundMethylphenidate (MPH) is a psychostimulant that exerts its pharmacological effects via preferential blockade of the dopamine transporter (DAT) and the norepinephrine transporter (NET), resulting in increased monoamine levels in the synapse. Clinically, methylphenidate is prescribed for the symptomatic treatment of ADHD and narcolepsy; although lately, there has been an increased incidence of its use in individuals not meeting the criteria for these disorders. MPH has also been misused as a “cognitive enhancer” and as an alternative to other psychostimulants. Here, we investigate whether chronic or acute administration of MPH in mice at either 1 mg/kg or 10 mg/kg, affects cell number and gene expression in the basal ganglia.
Methodology/Principal FindingsThrough the use of stereological counting methods, we observed a significant reduction (∼20%) in dopamine neuron numbers in the substantia nigra pars compacta (SNpc) following chronic administration of 10 mg/kg MPH. This dosage of MPH also induced a significant increase in the number of activated microglia in the SNpc. Additionally, exposure to either 1 mg/kg or 10 mg/kg MPH increased the sensitivity of SNpc dopaminergic neurons to the parkinsonian agent 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Unbiased gene screening employing Affymetrix GeneChip® HT MG-430 PM revealed changes in 115 and 54 genes in the substantia nigra (SN) of mice exposed to 1 mg/kg and 10 mg/kg MPH doses, respectively. Decreases in the mRNA levels of gdnf, dat1, vmat2, and th in the substantia nigra (SN) were observed with both acute and chronic dosing of 10 mg/kg MPH. We also found an increase in mRNA levels of the pro-inflammatory genes il-6 and tnf-α in the striatum, although these were seen only at an acute dose of 10 mg/kg and not following chronic dosing.
ConclusionCollectively, our results suggest that chronic MPH usage in mice at doses spanning the therapeutic range in humans, especially at prolonged higher doses, has long-term neurodegenerative consequences.

背景：哌甲酯（Methylphenidate, MPH）是一种精神兴奋剂，其药理作用通过选择性阻断多巴胺转运体（Dopamine Transporter, DAT）与去甲肾上腺素转运体（Norepinephrine Transporter, NET）实现，最终导致突触间隙中单胺类物质水平升高。临床上，哌甲酯被用于注意缺陷多动障碍（ADHD）与发作性睡病的对症治疗；但近来，不符合上述病症诊断标准的人群中，哌甲酯的使用比例呈上升趋势。此外，哌甲酯还被滥用作“认知增强剂”，或作为其他精神兴奋剂的替代用品。本研究旨在探究，以1 mg/kg或10 mg/kg剂量对小鼠进行急性或慢性哌甲酯给药，是否会影响其基底神经节的细胞数量与基因表达。
方法与主要结果：通过体视学计数方法，我们观察到，慢性给予10 mg/kg剂量哌甲酯后，小鼠黑质致密部（Substantia Nigra Pars Compacta, SNpc）内的多巴胺神经元数量显著减少（约20%）；该剂量的哌甲酯同时可诱导黑质致密部活化小胶质细胞数量显著增加。此外，以1 mg/kg或10 mg/kg剂量暴露于哌甲酯，均可提升黑质致密部多巴胺能神经元对帕金森病造模剂1-甲基-4-苯基-1,2,3,6-四氢吡啶（1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, MPTP）的敏感性。采用Affymetrix GeneChip® HT MG-430 PM进行无偏基因筛选，结果显示，分别给予1 mg/kg和10 mg/kg剂量哌甲酯的小鼠，其黑质（Substantia Nigra, SN）内分别有115个和54个基因的表达发生改变。以10 mg/kg剂量进行急性或慢性给药后，均可观察到黑质内gdnf、dat1、vmat2及th的mRNA水平下调。我们还发现，纹状体中的促炎基因il-6与tnf-α的mRNA水平升高，但该现象仅在10 mg/kg急性给药时出现，慢性给药时未观察到此变化。
结论：综上，本研究结果表明，以接近人类治疗剂量的哌甲酯对小鼠进行长期给药，尤其是高剂量长期给药时，会产生长期的神经退行性后果。