Research ReportmTOR and its downstream pathway are activated in the dorsal root ganglion and spinal cord after peripheral inflammation, but not after nerve injury☆
Introduction
Chronic pain, usually caused by inflammation or tissue or nerve injury, is a major public health problem worldwide. The development of optimal treatments for such pain has remained elusive because of the complex pathologic mechanisms involved. Changes in neuronal plasticity in spinal cord and dorsal root ganglion (DRG) are thought to contribute to the development and maintenance of chronic pain. Understanding the molecular mechanisms that underlie these changes could be helpful for developing novel pain treatments.
Mammalian target of rapamycin (mTOR) is a serine/threonine protein kinase expressed in the mammalian nervous system. Once mTOR binds to Raptor protein, it forms mTOR complex 1 and phosphorylates downstream effectors, such as p70 ribosomal S6 protein kinase 1 (S6K1), to govern translation of mRNA into protein (Swiech et al., 2008, Jaworski and Sheng, 2006, Hay and Sonenberg, 2004). The activity of mTOR can be modulated in response to a variety of stimuli, such as neurotransmitters, trophic factors, mitogens, hormones, cell energy status and cellular stress, ischemia, and heat shock (Swiech et al., 2008, Jaworski and Sheng, 2006). In turn, the activation of mTOR and its downstream effectors can influence numerous physiologic and pathologic processes in the nervous system (Swiech et al., 2008, Jaworski and Sheng, 2006). Recent studies showed that mTOR and its downstream effectors might be implicated in neuropathic pain, inflammatory pain, and cancer pain. Systemic or local administration of rapamycin, a specific inhibitor of mTOR, has been shown to alleviate mechanical hypersensitivity induced by intraplantar injection of carrageenan or capsaicin (inflammatory pain), peripheral nerve injury (neuropathic pain), and injection of prostate cancer cells into tibia (bone cancer pain) (Shih et al., 2012, Xu et al., 2011, Obara et al., 2011, Norsted et al., 2010, Geranton et al., 2009, Jimenez-Diaz et al., 2008, Price et al., 2007). Although mTOR and S6K1 are highly expressed in small DRG neurons and spinal cord dorsal horn neurons, their active (phosphorylated) forms are expressed at only very low levels under normal conditions (Xu et al., 2010). We and others have shown that peripheral noxious insults caused by intraplantar carrageenan or bone cancer lead to increases in phosphorylation of mTOR (p-mTOR) and S6K1 (p-S6K1) in rat spinal dorsal horn but not in DRGs (Shih et al., 2012, Xu et al., 2011). However, little is known about the role of mTOR in chronic inflammatory pain. Moreover, whether mTOR and its downstream effectors are activated in spinal cord and DRG after peripheral nerve injury is controversial (Melemedjian et al., 2011, Asante et al., 2010, Geranton et al., 2009).
In the present study, we compared the expression and activity of mTOR and S6K1 in DRG and spinal cord of rats during chronic inflammatory pain induced by intraplantar injection of complete Freund's adjuvant (CFA) and during chronic neuropathic pain caused by spinal nerve ligation (SNL).
Section snippets
mTOR and S6K1 activity in DRG and spinal cord under chronic inflammatory pain conditions
Consistent with previous studies (Yaster et al., 2011, Park et al., 2009, Zhang et al., 2003), CFA injection produced mechanical hypersensitivity, as evidenced by a reduction in paw withdrawal threshold (PWT), and thermal pain hypersensitivity, as evidenced by a reduction in paw withdrawal latency (PWL). Hypersensitivity developed only on the ipsilateral side, was present by 2 h after CFA injection, reached a peak level around day 1, and lasted for at least 7 days (Figs. 1a and b).
Discussion
Peripheral nerve injury and inflammation produce chronic pain hypersensitivities in rodent animal models that mimic clinical chronic neuropathic pain and chronic inflammatory pain, respectively. Understanding the mechanisms that cause pain hypersensitivity may lead to novel therapeutic strategies for its prevention and/or treatment. Although chronic neuropathic pain and inflammatory pain share some intracellular signaling pathways (Latremoliere and Woolf, 2010), each type of pain may also have
Animal preparation
Adult, male Sprague Dawley rats (250–300 g) were housed in cages on a standard 12:12 h light/dark cycle. Water and food were available ad libitum until rats were transported to the laboratory for experiments. The animals were used in protocols approved by the Animal Care and Use Committee at the Johns Hopkins University. Animal procedures were consistent with the ethical guidelines of the National Institutes of Health, the International Association for the Study of Pain, and the ethical
Acknowledgments
The authors thank Claire F. Levine, MS, for her editorial assistance. The authors do not have any conflicts of interest.
References (27)
- et al.
Mammalian target of rapamycin signaling in the spinal cord is required for neuronal plasticity and behavioral hypersensitivity associated with neuropathy in the rat
J. Pain
(2010) - et al.
Synaptic plasticity and central sensitization: author reply
J. Pain
(2010) - et al.
Dynamic temporal and spatial regulation of mu opioid receptor expression in primary afferent neurons following spinal nerve injury
Eur. J. Pain
(2011) - et al.
Systemic inhibition of the mammalian target of rapamycin (mTOR) pathway reduces neuropathic pain in mice
Pain
(2011) - et al.
Spinal cord NMDA receptor-mediated activation of mammalian target of rapamycin is required for the development and maintenance of bone cancer-induced pain hypersensitivities in rats
J. Pain
(2012) - et al.
Role of mTOR in physiology and pathology of the nervous system
Biochim. Biophys. Acta
(2008) - et al.
Expression and distribution of mTOR, p70S6K, 4E-BP1, and their phosphorylated counterparts in rat dorsal root ganglion and spinal cord dorsal horn
Brain Res.
(2010) - et al.
Effect of knock down of spinal cord PSD-93/chapsin-110 on persistent pain induced by complete Freund's adjuvant and peripheral nerve injury
Pain
(2003) - et al.
mTOR as a therapeutic target in patients with gastric cancer
Int. J. Cancer
(2012) - et al.
Kidney transplant histology after one year of continuous therapy with sirolimus compared with tacrolimus
Transplantation
(2008)
Mammalian target of rapamycin inhibition as a therapeutic strategy in the management of urologic malignancies
Mol. Cancer Ther.
A rapamycin-sensitive signaling pathway is essential for the full expression of persistent pain states
J. Neurosci.
Peripheral nerve injury up-regulates expression of interactor protein for cytohesin exchange factor 1 (IPCEF1) mRNA in rat dorsal root ganglion
Naunyn Schmiedebergs Arch. Pharmacol.
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This work was supported by Grants (NS072206, NS058886, and DA033390) from the National Institutes of Health; Mr. David Koch and the Patrick C. Walsh Prostate Cancer Research Fund; the Rita Allen Foundation; and the Blaustein Pain Research Fund.