Evidence shows that glial activation increases the expressio
Evidence shows that glial activation increases the expression of cytokines and chemokines in Parkinson's disease, which precede the degeneration of the substance nigra (Halliday and Stevens, 2011, Tansey and Goldberg, 2010). The use of non-steroidal anti-inflammatory drugs (except aspirin) can prevent or delay the onset of Parkinson's (see review in Rocha et al., 2015) and the use of natural molecules with antioxidant and anti-inflammatory properties can protect dopaminergic neurons (Magalingam et al., 2015, Ojha et al., 2016, Sandoval-Avila et al., 2016). Recently, Bayliss et al. (2016) found that circulating ghrelin, which is elevated during caloric restriction, plays a protective role in the nigrostriatal system via enhanced AMPK activity. According to the authors, caloric restriction in ghrelin WT mice attenuated the MPTP-induced loss of substantia nigra dopamine neurons and striatal dopamine turnover, but these effects were abolished in ghrelin knockout (KO) mice, demonstrating that ghrelin has a neuroprotective effect. Moreover, treatment with ghrelin elevated p-AMPK and ACC levels in the striatum of AMPK WT, but not KO mice, suggesting that AMPK is a target for the ghrelin-induced neuroprotective effect. Likewise, chronic ghrelin treatment to AMPK WT, but not KO mice reduced the activation of microglia and astrocytes. On the other hand, MPTP-induced neuronal damage was accompanied by an increase in AMPK, but AMPK overexpression by transfection into SH-SY5Ycells exhibited a neuroprotective function (Choi et al., 2010). Equally, pAMPK was elevated in cytotoxicity rotenone-induced in Mes 23.5 NG,NG-dimethyl-L-Arginine hydrochloride (dopaminergic cell line), but the inhibition of AMPK lowered the accumulation of rotenone-induced synuclein and promoted cell death (Hou et al., 2015). These results could be related to pAMPK up-regulation in neurons that are damaged in a compensatory protective loop as a strategy to increase the energy supply and neuronal viability. More recently, Lu et al. (2016) used a MPTP plus probenecid (MPTP/p) mouse model of Parkinson's to explore the therapeutic effect of metformin on dopaminergic neuronal degeneration. The results demonstrated that metformin inhibited microglial activation as well as decreased both NF-κBp65 nuclear translocation and the activation of NLRP3 inflammasome. Moreover, metformin reduced the transcription of pro-inflammatory cytokines (TNF-α and IL-6), restored the transcription of anti-inflammatory cytokines [IL-10, transforming growth factor-β (TGF-β) and IL-4], preserved the dopaminergic system and attenuated α-synuclein accumulation and motor symptoms. The reduction in the symptoms of Parkinson's disease was attributed to AMPK activation. In vitro, MPP+ promoted cell death and increased levels of pAMPK and ROS. Since MPTP or MPP+ are mitochondrial complex I inhibitors, these molecules could also promote the increase in AMPK levels through a compensative auto-regulatory mechanism. Moreover, co-treatment with MPP+ and metformin enhanced AMPK levels even more and also elevated both AMPK-mediated autophagy and mitochondrial ROS clearance as well as attenuated dopaminergic neuron apoptosis. These results were confirmed by the suppression of the protective effects of metformin in SH-SY5Y cells after Compound C treatment. Thus, metformin can enhance the AMPK-autophagy pathway and, in turn, reduces ROS production and inhibits the activation of inflammasome. However, some authors have hypothesized that treatment with metformin may be considered as a risk factor for the development of Parkinson's disease. In a study conducted by Ismaiel et al. (2016), metformin reduced microglial activation as well as TNF-α, IL-1β and iNOS mRNAs levels, but did not prevent the negative effect of MPTP on dopaminergic neurons, as evaluated by TH staining as well as the level of dopamine and its metabolite DOPAC measured in the striatum. In fact, metformin exacerbated dopaminergic harm in response to MPTP. Moreover, in vitro experiments on the viability of N27 dopaminergic neurons using the MTT assay showed that metformin not only failed to protect neurons, but even increased the harm. The authors suggest that neuronal death is attributed to the inhibition of the mitochondrial complex I by both MPTP and metformin. However, a point to consider is that metformin also can exert effects in an AMPK-independent manner, which makes its evaluation more complex (Łabuzek et al., 2010). Furthermore, the absence of AMPK inhibitors in the study impedes the inference that cell damage occurred in an AMPK-dependent manner.