PPARγ activation blocks development and reduces established neuropathic pain in rats
Highlights
► PPARγ activation blocks development of and established neuropathic pain. ► Inhibition of neuropathic pain persists long after cessation of pioglitazone. ► Pioglitazone reduces spinal microglial and astrocyte activation. ► Pioglitazone reduces light touch stimulus-induced expression of p-ERK in dorsal horn. ► Anti-hyperalgesic effects extended to multiple stimulus modalities in the absence of side effects.
Introduction
Neuropathic pain is defined as the pain caused by a lesion or disease of the somatosensory system (Jensen et al., 2011). Nerve injury produces numerous neurobiological events in the peripheral and central nervous system that contribute to chronic pain (Taylor, 2009). The mechanisms in the dorsal horn that underlie enhanced pain signaling include post-translational modifications (Woolf and Mannion, 1999) and microglia and/or astrocyte activation (Milligan and Watkins, 2009). For example, nerve injury induces activation of mitogen-activated protein kinases (MAPKs), as reflected by the phosphorylation of extracellular signal-regulated kinase (ERK) and p38 in spinal microglia (Ji et al., 2009; Jin et al., 2003; Zhuang et al., 2005). Such mechanisms provide multiple pharmacological targets for the treatment of chronic pain, but further research is required to improve neuropathic pain relief beyond currently available analgesic drugs, which yield only limited beneficial outcomes and produce serious side effects (Finnerup et al., 2010).
As insulin sensitizers, synthetic peroxisome proliferator-activated receptor gamma (PPARγ) agonists of the thiazolidinedione (TZD) class, such as rosiglitazone and pioglitazone, remain an important pharmacotherapeutic class for the treatment of type II diabetes (Evans et al., 2004; Martens et al., 2002). Beginning with our finding that intrathecal administration of rosiglitazone reduced behavioral signs of allodynia and hyperalgesia in the spared nerve injury (SNI) model of neuropathic pain (Churi et al., 2008), a growing number of studies report that systemic administration of TZDs reduce peripheral neuropathic pain (Jia et al., 2010; Maeda et al., 2008; Takahashi et al., 2011). PPARγ immunoreactivity is found in spinal cord and brain (Maeda et al., 2008; Moreno et al., 2004; Victor et al., 2006; Zhao et al., 2006). Thus, the translational potential of future TZD drugs for neuropathic pain is particularly exciting. Pioglitazone has a particular advantage as an experimental drug in that it readily crosses the blood brain barrier (Maeshiba et al., 1997) and thus has access to the PPARγ that is expressed in the dorsal horn of the spinal cord (Churi et al., 2008; Shibata et al., 2008). Furthermore, pioglitazone continues to be FDA-approved as a one-year therapy for diabetes. However, the existing literature provides only limited or conflicting information regarding the mechanism of anti-allodynic and anti-hyperalgesic action of TZDs such as pioglitazone (Jia et al., 2010; Maeda et al., 2008; Takahashi et al., 2011). Therefore, the present study evaluated timing, duration of action, and PPARγ-antagonist reversibility following oral and/or intraperitoneal administration. Most importantly, we tested the hypothesis that chronic administration of pioglitazone acts as a PPARγ agonist to inhibit the induction and/or maintenance phases of neuropathic pain.
To address this question, we used the intrathecal route to administer the selective PPARγ antagonist GW9662 before systemic pioglitazone. GW9662 has nanomolar affinity for PPARγ in binding experiments, with 10- and 600-fold less potency at PPARα and PPARδ, respectively (Leesnitzer et al., 2002). In addition, we evaluated the effect of pioglitazone on phosphorylation of extracellular signal-regulated kinase (p-ERK) in the superficial dorsal horn. p-ERK has emerged as a marker of increased neuronal activation in response to peripheral stimulation in inflammatory and neuropathic pain models (Gao and Ji, 2009; Ji et al., 1999; Zhuang et al., 2005).
Events that induce hyperalgesia, such as nerve injury, activate glia in the spinal cord (Ji and Suter, 2007; Milligan and Watkins, 2009; Scholz and Woolf, 2007; Svensson and Brodin, 2010). Spinal glia express a variety of receptors for neurotransmitters and neuromodulators, and produce and release numerous signaling molecules that could ultimately contribute to central sensitization and chronic pain (Milligan and Watkins, 2009). Because PPARγ agonists reduce glial activation in vitro (Bernardo et al., 2000; Jiang et al., 1998; Ricote et al., 1998) and in vivo in brain (Heneka et al., 2005; Petrova et al., 1999; Schintu et al., 2009) and spinal cord (Sauerbeck et al., 2011; Shibata et al., 2008), it is conceivable that they would have similar effects in nociceptive regions of the dorsal horn, ultimately leading to reductions in pain. Therefore, we administered pioglitazone and used antibodies against cd11b and glial fibrillary acidic protein (GFAP) to assess the spinal expression of microglia and astrocytes, respectively. We also measured p-p38 which is predominantly expressed in microglia (Ji and Suter, 2007).
We found that chronic pioglitazone reduces spinal glial and ERK activation, and operates at PPARγ to block the development and maintenance of neuropathic pain. The current results substantially extend our understanding of the pharmacodynamics and mechanism of the anti-allodynic and anti-hyperalgesic actions of TZDs, and, most importantly, establish spinal PPARγ systems as a promising pharmacotherapeutic target for the treatment of neuropathic pain using new PPARγ agonists.
Section snippets
Animals
Male Sprague–Dawley rats (Charles River Laboratories, Wilmington, MA) weighing 200–300 g at the time of surgery were housed 2–3 per bedded cage on a 12-h light/dark cycle (7am/7pm) in a temperature (68–72 °F) and humidity-controlled room with food and water provided ad libitum. All efforts were made to minimize animal suffering, to reduce the number of animals used, and to utilize alternatives to in vivo techniques, in accordance to the guidelines set forth by the National Institutes of Health
Pioglitazone reduces the development of nerve injury-induced hypersensitivity
Previous studies indicate that oral gavage of pioglitazone reduced the heat hyperalgesia and/or mechanical allodynia associated with either L5 spinal nerve transection in rats or partial sciatic nerve ligation (PSNL) in mice (Jia et al., 2010; Maeda et al., 2008). However, there were several limitations inherent in these studies: Tests were limited to 1–2 week periods of drug administration; they did not evaluate two key modalities of neuropathic pain (hypersensitivity to cold temperature and
Discussion
A growing number of studies report that systemic administration of thiazolidinediones (TZDs) reduce peripheral neuropathic pain (Jia et al., 2010; Maeda et al., 2008; Takahashi et al., 2011). Here, we report that oral or twice-daily intraperitoneal pioglitazone, beginning before SNI and continuing for several weeks, dose-dependently prevented behavioral signs of mechanical and cold hypersensitivity.
Acknowledgments
Supported by 5R01NS62306 and 5K02DA19656 to BKT.
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