Neuropathic pain-induced depressive-like behavior and hippocampal neurogenesis and plasticity are dependent on TNFR1 signaling

Abstract

Patients suffering from neuropathic pain have a higher incidence of mood disorders such as depression. Increased expression of tumor necrosis factor (TNF) has been reported in neuropathic pain and depressive-like conditions and most of the pro-inflammatory effects of TNF are mediated by the TNF receptor 1 (TNFR1). Here we sought to investigate: (1) the occurrence of depressive-like behavior in chronic neuropathic pain and the associated forms of hippocampal plasticity, and (2) the involvement of TNFR1-mediated TNF signaling as a possible regulator of such events. Neuropathic pain was induced by chronic constriction injury of the sciatic nerve in wild-type and TNFR1^−/− mice. Anhedonia, weight loss and physical state were measured as symptoms of depression. Hippocampal neurogenesis, neuroplasticity, myelin remodeling and TNF/TNFRs expression were analyzed by immunohistochemical analysis and western blot assay.

We found that neuropathic pain resulted in the development of depressive symptoms in a time dependent manner and was associated with profound hippocampal alterations such as impaired neurogenesis, reduced expression of neuroplasticity markers and myelin proteins. The onset of depressive-like behavior also coincided with increased hippocampal levels of TNF, and decreased expression of TNF receptor 2 (TNFR2), which were all fully restored after mice spontaneously recovered from pain. Notably, TNFR1^−/− mice did not develop depressive-like symptoms after injury, nor were there changes in hippocampal neurogenesis and plasticity.

Our data show that neuropathic pain induces a cluster of depressive-like symptoms and profound hippocampal plasticity that are dependent on TNF signaling through TNFR1.

Introduction

Over half of all patients who suffer from neuropathic pain develop mood disorders such as depression and anxiety (Maletic and Raison, 2009, McWilliams et al., 2003), but the mechanisms underlying this comorbidity are not fully understood. Accumulating evidence suggests a role for the immune system in the etiology of depression (Eyre and Baune, 2012). Elevated levels of immune mediators such as TNF, have been detected in depressed patients (Mikova et al., 2001, Tuglu et al., 2003), while in rodents high levels of cytokines induce a depressive-like behavior, known as “sickness behavior” (Hart, 1988, Kaster et al., 2012). This condition can be reliably reproduced with the administration of cytokines or cytokine-inducers (Harrison et al., 2009, Yirmiya, 1996), and blocked by cytokine antagonists, or anti-inflammatory cytokines (Dantzer, 2001, Kent et al., 1992, Shamash et al., 2002). Moreover, genetically modified mice that do not express TNF receptors (TNFRs) are more resistant to the development of depressive behavior under stressful conditions, while TNF administration renders mice more susceptible to depression (Simen et al., 2006). It has been shown that antidepressants can reduce plasma TNF concentration (Kubera et al., 2005, Yirmiya et al., 1999), and in clinical trials, in which TNFRs antagonists were used for the treatment of immune pathologies, a significant improvement of depressive symptoms was observed (Bos and de Korte, 2006, Ertenli et al., 2012, Tyring et al., 2006).

TNF signals via 2 distinct receptors which often mediate opposing biological functions: the pro-inflammatory/pro-neurodegenerative/pro-demyelinanting TNF receptor 1 (TNFR1/p55) and the likely neuroprotective TNF receptor 2 (TNFR2/p75) (Baud and Karin, 2001, Brambilla et al., 2011, MacEwan, 2002). Interestingly, TNF has been proven to have a key role in the development of neuropathic pain (George et al., 2004, Martuscello et al., 2012), which has been associated to its action through TNFR1 (Schafers et al., 2002, Vogel et al., 2006).

The hippocampus, a central component of the limbic system, is a crucial mood-regulating region of the brain, also involved in the processing of nociception (Mutso et al., 2012). With the discovery of new neuron formation in this area of the adult brain, significant emphasis has been ascribed to the role of the neurogenic process in mood regulation and impairment of adult hippocampal neurogenesis has been linked to the development of depression (Sahay and Hen, 2007). However, other neuroplastic changes such as reduced spine density and dendritic retraction, were previously shown to occur at this level in animal models of depression or pain (Duman and Charney, 1999, Kodama et al., 2007, Watanabe et al., 1992) and, as with the neurogenic process, these alterations can be reverted by treatment with antidepressants as animals recover from depressive-like symptoms (Reinés et al., 2008, Warner-Schmidt and Duman, 2006). It is noteworthy that impairments in brain white matter have been described in psychiatric diseases such as schizophrenia and depression (Kyriakopoulos et al., 2009, Mettenburg et al., 2012), and specifically have been found to be associated with the limbic system in the melancholic subtype of major depressive disorder (Korgaonkar et al., 2011). Interestingly, Zeng et al. (2012) showed that white matter volume is normalized by antidepressant treatment in patients with major depression. So far little is known about the mechanisms implicated in white matter impairments that occurs in depressed patients. Conversely, demyelinating disorders characterized by myelin loss show co-morbidity with depression (Arnett et al., 2008), yet, the possible contribution of myelin remodeling as part of the hippocampal plasticity that occurs in depression has not been addressed.

Notably, it has been shown that TNF can be detrimental for the survival of the new hippocampal neurons (Cacci et al., 2005, Monje et al., 2003) and that TNF receptors have a differential actions in the modulation of hippocampal neurogenesis; TNFR1 acts as a suppressor of adult neurogenesis, whereas the absence TNFR2 results in reduced hippocampal neurogenesis (Chen and Palmer, 2013, Iosif et al., 2006). Interestingly, TNF has been shown to have a role in the pathophysiology of autoimmune demyelinating diseases such as multiple sclerosis and experimental autoimmune encephalomyelitis (EAE) (Finsen et al., 2002) and the role of TNFR1-mediated TNF signaling in causing demyelination has been widely investigated by different authors (Arnett et al., 2001, Eugster et al., 1999, Probert et al., 2000). Finally, we and others have demonstrated that signaling mediated by transmembrane TNF, occurring mainly through TNFR2, has been proven to be essential for axon and myelin preservation and to promote remyelination (Bracchi-Ricard et al., 2013, Brambilla et al., 2011, Taoufik et al., 2011).

Based upon this evidence we sought to determine whether neuropathic pain-associated depression is related to hippocampal neuroplasticity and myelin alterations, and whether TNF signaling through TNFR1 plays a role in these events. We hypothesized that chronic neuropathic pain would be associated with the occurrence of depressive-like symptoms and that this would be mediated by hippocampal TNF/TNFR1 signaling resulting in impairment of hippocampal neurogenesis and neuroplasticity. Given the role of TNF/TNFR1 in demyelinating pathologies we also tested the hypothesis of myelin remodeling occurring in the hippocampus of mice suffering from neuropathic-pain-induced depression. To this purpose we used wild-type and TNFR1^−/− mice to investigate: (1) the occurrence of depressive-like symptoms, such as anhedonia, loss of body weight gain and self-neglect in mice with neuropathic pain; (2) hippocampal neurogenesis, neuronal plasticity and myelin remodeling and (3) hippocampal TNF and TNFRs expression at the onset and recovery from neuropathic pain.

Our data show a temporal relationship between neuropathic pain and the occurrence of depressive-like symptoms as well as structural neuroplastic and white matter impairments of the hippocampus. Moreover, this is the first report to show that neuropathic pain-induced depression is associated with hippocampal TNF activity through TNFR1 and that this correlates with profound hippocampal plasticity.