Clinical and preclinical studies have indicated that chronic methamphetamine (MA) use is associated with extensive neurodegeneration, psychosis, and cognitive impairment. Evidence from animal models has suggested a considerable role of excess dopamine or glutamate, oxidative stress, neuroinflammation, and apoptosis in MA-induced neurotoxicity, and that protein kinase Cδ might mediate the interaction among these factors. In addition, the relatively long-lasting and recurrent nature of MA psychosis has been reproduced in animals treated with various dosing regimens of MA, which have shown behavioral sensitization, sociability deficits, and impaired prepulse inhibition. Genetic predisposition as well as dopaminergic and glutamatergic alterations might be important in the development of MA psychosis. Neuroimaging studies have identified functional and morphological changes related to the cognitive dysfunction shown in chronic MA users. Failure in the task-evoked phosphorylation of extracellular signal-related kinase likely underlies MA-induced memory impairment. Recent progress has suggested certain roles of oxidative stress and neuroinflammation in the psychosis and cognitive deficits induced by repeated low doses of MA. This review provides a comprehensive description of pertinent findings from human and animal studies, with an emphasis on the current understanding of the underlying mechanisms of MA neuropsychotoxicity and its relevance to Parkinson's disease or schizophrenia.

Accumulating evidence has suggested that repeated treatment with methamphetamine (MA) resulted in cognitive impairments. Importantly, we show that selective upregulation of protein kinase Cδ (PKCδ) in the prefrontal cortex (PFC) of wild-type mice persisted for 28 days post withdrawal of MA. On day 28, the MA-induced increase in phospho-PKCδ expression and decrease in phospho-ERK expression were significantly attenuated by both the Src inhibitor PP2 and the dopamine D1 receptor antagonist SCH 23390. However, neither protein kinase A inhibitor H89 nor calmodulin-dependent protein kinase II inhibitor KN93 attenuated MA-induced alterations in phospho-PKCδ expression and phospho-ERK expression. Since PKCδ knockout (KO) significantly increased the expression of glutathione peroxidase (GPx)-1, we also utilized GPx-1 KO and GPx-1-overexpressing transgenic (GPx-1 TG) mice. Repeated MA treatment induced cognitive impairment, as assessed by the novel object recognition test. Moreover, the extent of cognitive impairment correlated with the extent of increased phospho-PKCδ expression and decreased GPx1 expression. In the absence of MA, exposure to novel objects increased phospho-ERK and GPx-1 expression in the PFC; however, these expression levels were decreased in the presence of MA. PKCδ KO and GPx-1 TG mice each exhibited significantly attenuated MA-induced decreases in phospho-ERK and GPx-1 expression. Consistently, PKCδ inhibition induces GPx/GSH-dependent antioxidant systems. More importantly, the antipsychotic drug clozapine significantly protected against cognitive impairment and was associated with alterations in phospho-ERK and phospho-PKCδ expression. However, GPx-1 KO potentiated MA-induced cognitive deficits and alterations in phospho-ERK and phospho-PKCδ expression. These results suggest that MA induces cognitive impairment by inhibiting ERK signaling, activating PKCδ, and inactivating GPx-1 by upregulating Src kinase or the D1 receptor. They also suggest that clozapine requires activation of ERK signaling via positive modulation between the phospho-PKCδ and GPx-1 genes to restore cognitive function.

Selegiline is a monoamine oxidase-B (MAO-B) inhibitor with anti-Parkinsonian effects, but it is metabolized to amphetamines. Since another MAO-B inhibitor N-Methyl, N-propynyl-2-phenylethylamine (MPPE) is not metabolized to amphetamines, we examined whether MPPE induces behavioral side effects and whether MPPE affects dopaminergic toxicity induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Multiple doses of MPPE (2.5 and 5 mg/kg/day) did not show any significant locomotor activity and conditioned place preference, whereas selegiline (2.5 and 5 mg/kg/day) significantly increased these behavioral side effects. Treatment with MPPE resulted in significant attenuations against decreases in mitochondrial complex I activity, mitochondrial Mn-SOD activity, and expression induced by MPTP in the striatum of mice. Consistently, MPPE significantly attenuated MPTP-induced oxidative stress and MPPE-mediated antioxidant activity appeared to be more pronounced in mitochondrial-fraction than in cytosolic-fraction. Because MPTP promoted mitochondrial p53 translocation and p53/Bcl-xL interaction, it was also examined whether mitochondrial p53 inhibitor pifithrin-μ attenuates MPTP neurotoxicity. MPPE, selegiline, or pifithrin-μ significantly attenuated mitochondrial p53/Bcl-xL interaction, impaired mitochondrial transmembrane potential, cytosolic cytochrome c release, and cleaved caspase-3 in wild-type mice. Subsequently, these compounds significantly ameliorated MPTP-induced motor impairments. Neuroprotective effects of MPPE appeared to be more prominent than those of selegiline. MPPE or selegiline did not show any additional protective effects against the attenuation by p53 gene knockout, suggesting that p53 gene is a critical target for these compounds. Our results suggest that MPPE possesses anti-Parkinsonian potentials with guaranteed behavioral safety and that the underlying mechanism of MPPE requires inhibition of mitochondrial oxidative stress, mitochondrial translocation of p53, and pro-apoptotic process.

Escalating evidence suggests that the impairment of glutathione (GSH)-dependent systems is one of the etiologic factors of schizophrenia. GSH is an important substrate of glutathione peroxidase (GPx). Among GPx isozymes, selenium-dependent GPx (GPx-1) is recognized as a major type, and therefore, this study investigates the role of the GPx-1 gene in abnormal behaviors induced by repeated phencyclidine (PCP) treatment using GPx-1 knockout (KO) and overexpressing transgenic (GPx-1 TG) mice. PCP-induced abnormal behaviors were more pronounced in GPx-1 KO mice than abnormal behaviors in wild-type (WT) mice, and the abnormal behaviors were less pronounced in GPx-1 TG mice than abnormal behaviors in non-TG mice. PCP treatment significantly reduced GSH levels and enhanced oxidative burdens in the prefrontal cortex of the test animals. In addition, PCP treatment significantly upregulated the nuclear translocations of nuclear factor erythroid-2-related factor 2 (Nrf2) and nuclear factor kappa-B (NF-κB) p65, as well as their DNA binding activities and γ-glutamylcysteine (GCL) mRNA expression in WT and non-TG mice. However, GPx-1 KO abolished this upregulation system. In contrast, genetic overexpression of GPx-1 further upregulated Nrf2-dependent GSH synthetic system, but downregulated NF-κB p65 activity in the presence of PCP. Clozapine, an antipsychotic, significantly upregulated GPx-1 and Nrf2-dependent GSH synthetic systems in the presence of PCP, but failed to affect NF-κB p65 activity. Our results suggest that interactive modulations between the GPx-1 gene and Nrf2-dependent GSH induction are critical for attenuating PCP-induced abnormal behaviors in mice.

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