Published On: Mon, Jan 14th, 2019

Treatment with methyl-{beta}-cyclodextrin prevents mechanical allodynia in resiniferatoxin neuropathy in a mouse model [RESEARCH ARTICLE]


A specialized microdomain which has cholesterol-rich membrane regions contains several transmembrane molecules that modulate cellular physiology. Evidence suggests that microdomains serve as organizing centers and are correlated to neurodegeneration (Sonnino et al., 2014), peripheral neuropathy (Gambert et al., 2017; Lee et al., 2014), neuronal interactions (Leyton and Hagood, 2014; Marchenkova et al., 2016) and synaptic transmission (De Chiara et al., 2013). Alteration of the membrane composition of a microdomain is associated with the pathogenesis of neurodegeneration (Gambert et al., 2017; Lee et al., 2014; Sonnino et al., 2014). Disrupting the membrane integrity causes distinct effects such as reversing the cytotoxicity of antitumor drugs (Adinolfi et al., 2013), antagonizing hyperalgesia (Dina et al., 2005) and inhibiting endocannabinoid-mediated analgesic systems (Rossi et al., 2012). These effects imply that microdomains may affect neuronal regulation, particularly in neuronal antinociception. Notably, the transmembrane isoform of prostatic acid phosphatase (PAP) was previously documented in microdomains (Quintero et al., 2007); PAP has ectonucleotidase properties (Sowa et al., 2009) that can hydrolyze extracellular adenosine monophosphate (AMP) to the antinociceptive agonist adenosine (Street and Zylka, 2011; Zylka et al., 2008). The antinociceptive effect of PAP that prevents pain hypersensitivity has been thoroughly documented, and our previous research confirmed that PAP neuropathology results in the loss of antinociception; that is, injured PAP(+) neurons mediate pain hypersensitivity (Wu et al., 2016) through disorder of adenosine signaling (Kan et al., 2018). However, the mechanism involved in this effect remains uncertain and requires further investigation. Research is necessary to determine whether a transmembrane molecule exists that colocalizes and interacts with PAP in an integrity of membrane structure to modulate PAP-mediated antinociception. If one does exist, transient receptor potential vanilloid subtype 1 (TRPV1) is a candidate because PAP mediates antinociception by reducing TRPV1 activity (Sowa et al., 2010), and TRPV1-mediated nociception requires TRPV1 on membrane integrity (Marchenkova et al., 2016; Saghy et al., 2015; Szőke et al., 2010).

TRPV1 is a non-selective ion channel and a polymodal nociceptor that responds to thermal nociception (Caterina et al., 2000; Caterina et al., 1997); depletion of TRPV1(+) neurons using the selective neurotoxic agent resiniferatoxin (RTX) leads to thermal hypoalgesia (Hsieh et al., 2012b; Karai et al., 2004). We established a mouse model of pure small-fiber neuropathy that causes mechanical allodynia in addition to reducing intraepidermal nerve fibers (IENFs) and inducing thermal hypoalgesia (Hsieh et al., 2012a;  2008, 2012b; Pan et al., 2003). On the basis of this RTX neuropathy model, we demonstrated that mechanical allodynia and thermal hypoalgesia were induced concurrently through distinct pathways (Hsieh et al., 2012a, b; Lin et al., 2013) and by different neurotrophin-dependent receptors (Hsieh et al., 2018). In addition, PAP was determined to mediate antinociception through hydrolysis of phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2], which was found to reduce TRPV1-mediated nociception (Sowa et al., 2010). By contrast, the depletion of TRPV1(+) neurons has been demonstrated to induce ATP-sensitized P2X3 (Hsieh et al., 2012a), which has been demonstrated to colocalize with PAP (Wu et al., 2016). Collectively, these findings suggest that TRPV1(+) neuron-dependent neuropathic manifestation involves the close interaction of TRPV1 and PAP because of colocalization and that these two molecules are similarly neurophysiologically modulated (i.e. nerve growth factor-mediated tyrosine receptor kinase A signaling) (Wu et al., 2016). In this study, we investigated the following concerns: (1) whether PAP and TRPV1 that are colocalized in the same cholesterol-rich microdomain modulate antinociception and nociceptive transduction and (2) the consequences of disrupting the integrity of a microdomain containing PAP and TRPV1 in RTX neuropathy.

We conducted immunohistochemistry on the dorsal root ganglion (DRG) neurons and performed pharmacological interventions with methyl-β-cyclodextrin (MβC) to deplete membrane cholesterol contents in RTX neuropathy (Fig. 1). This study demonstrated that MβC-mediated cholesterol depletion preserved PAP-mediated antinociception and that depletion of TRPV1(+) neurons mediated nerve degeneration leading to pain hypersensitivity through downregulation of the PAP antinociceptive effect.

Fig. 1.

Fig. 1.

Schedules of methyl-β-cyclodextrin (MβC) administration in resiniferatoxin (RTX) neuropathy. MβC was administered in four doses through an intrathecal lumbar puncture (1 µg/5 µl, cumulative 4 µg per mouse) in different administered protocols described in the Materials and Methods. The arrowhead indicates RTX administration, and the arrows represent the time point of MβC administration. Behavior was tested at day 7 (RTXd7), RTXd14 and RTXd21 after RTX neuropathy. Schedules of MβC administration in RTX neuropathy for two protocols: (A) RTX mice that received MβC at RTXd0, RTXd1, RTXd3 and RTXd5 were assigned to the MβC pre-RTX group; (B) RTX mice that received MβC administered at RTXd7, RTXd9, RTXd11 and RTXd13 after RTX neuropathy were assigned to the MβC post-RTX group. The two control groups were as following: (C) naïve mice that received MβC were a positive control (MβC alone group), and (D) RTX mice that received saline were the negative control (Sal+RTX group).

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