Endogenous interleukin-6 contributes to hypersensitivity to cutaneous stimuli and changes in neuropeptides associated with chronic nerve constriction in mice

Interleukin-6 (IL6) and cellular response to facial nerve injury: effects on lymphocyte recruitment, early microglial activation and axonal outgrowth in IL6-deficient mice

Nerve injury triggers numerous changes in the injured neurons and surrounding non-neuronal cells. Of particular interest are molecular signals that play a role in the overall orchestration of this multifaceted cellular response. Here we investigated the function of interleukin-6 (IL6), a multifunctional neurotrophin and cytokine rapidly expressed in the injured nervous system, using the facial axotomy model in IL6-deficient mice and wild-type controls. Transgenic deletion of IL6 caused a massive decrease in the recruitment of CD3-positive T-lymphocytes and early microglial activation during the first 4 days after injury in the axotomized facial nucleus. This was accompanied by a more moderate reduction in peripheral regeneration at day 4, lymphocyte recruitment (day 14) and enhanced perikaryal sprouting (day 14). Motoneuron cell death, phagocytosis by microglial cells and recruitment of granulocytes and macrophages into injured peripheral nerve were not affected. In summary, IL6 lead to a variety of effects on the cellular response to neural trauma. However, the particularly strong actions on lymphocytes and microglia suggest that this cytokine plays a central role in the initiation of immune surveillance in the injured central nervous system.

Intrathecal interleukin-1 receptor antagonist in combination with soluble tumor necrosis factor receptor exhibits an anti-allodynic action in a rat model of neuropathic pain

The expression of interleukin-1beta and tumor necrosis factor has previously been shown to be up-regulated in the spinal cord of several rat mononeuropathy models. This present study was undertaken to determine whether blocking the action of central interleukin-1beta and tumor necrosis factor attenuates mechanical allodynia in a gender-specific manner in a rodent L5 spinal nerve transection model of neuropathic pain, and whether this inhibition occurs via down-regulation of the central cytokine cascade or blockade of glial activation. Interleukin-1 receptor antagonist or soluble tumor necrosis factor receptor was administered intrathecally via lumbar puncture to male Holtzman rats in a preventative pain strategy, in which therapy was initiated 1h prior to surgery. Administration of soluble tumor necrosis factor receptor attenuated mechanical allodynia, while interleukin-1 receptor antagonist alone was unable to decrease allodynia. Interleukin-1 receptor antagonist in combination with soluble tumor necrosis factor receptor, administered to both male and female rats in a preventative pain strategy, significantly reduced mechanical allodynia in a dose-dependent manner (P<0.01). The magnitude of attenuation in allodynia was similar in both males and females. Immunohistochemistry on L5 spinal cord revealed similar astrocytic and microglial activation regardless of treatment. At days 3 and 7 post-transection, animals receiving daily interleukin-1 receptor antagonist in combination with soluble tumor necrosis factor receptor exhibited significantly less interleukin-6, but not interleukin-1beta, in the L5 spinal cord compared to vehicle-treated animals. In an existing pain paradigm, in which treatment was initiated on day 7 post-transection, interleukin-1 receptor antagonist in combination with soluble tumor necrosis factor receptor attenuated mechanical allodynia (P<0.05) in male rats. These findings further support a role for central interleukin-1beta and tumor necrosis factor in the development and maintenance of neuropathic pain through induction of a proinflammatory cytokine cascade.

Targeted disruption of the galanin gene reduces the number of sensory neurons and their regenerative capacity

The neuropeptide galanin is expressed developmentally in the dorsal root ganglion (DRG) and is rapidly up-regulated 120-fold after peripheral nerve section in the adult. Here we report that adult mice carrying a loss-of-function mutation in the galanin gene have a 13% reduction in the number of cells in the DRG associated with a 24% decrease in the percentage of neurons that express substance P. These deficits are associated with a 2.8- and 2.6-fold increase in the number of apoptotic cells in the DRG at postnatal days 3 and 4, respectively. After crush injury to the sciatic nerve, the rate of peripheral nerve regeneration is reduced by 35% with associated long-term functional deficits. Cultured DRG neurons from adult mutant mice demonstrate similar deficits in neurite number and length. These results identify a critical role for galanin in the development and regeneration of sensory neurons.

Intrathecal anti-IL-6 antibody and IgG attenuates peripheral nerve injury-induced mechanical allodynia in the rat: possible immune modulation in neuropathic pain

Interleukin-6 (IL-6) is a pleiotrophic cytokine with a diverse range of actions including the modulation of the peripheral and central nervous system. We have previously shown significant IL-6 protein and messenger RNA elevation in rat spinal cord following peripheral nerve injury that results in pain behaviors suggestive of neuropathic pain. These spinal IL-6 levels correlated directly with the mechanical allodynia intensity following nerve injury. In the current study, we sought to determine whether it is possible to attenuate mechanical allodynia and/or alter spinal glial activation resulting from peripheral nerve injury by specific manipulation of IL-6 with neutralizing antibodies or by global immune modulation utilizing immunogamma-globulin (IgG). Effects of peripheral administration of normal goat IgG and intrathecal (i.t.) administration of IL-6 neutralizing antibody, normal goat or normal rat IgG on mechanical allodynia associated with L5 spinal nerve transection were compared. Spinal glial activation was assessed at day 10 post surgery by immunohistochemistry. Low dose (0.01-0.001 microg) goat anti-rat IL-6 i.t. administration (P=0.025) significantly decreased allodynia and trended towards significance at the higher dose (0.08 microg to 0.008 microg, P=0.062). Low doses (0.01-0.001 microg) i.t. normal goat and rat IgG significantly attenuated mechanical allodynia, but not at higher doses (0.08-0.008 microg; P=0.001 for both goat and rat IgG). Peripherally administered normal goat IgG (30 or 100 mg/kg) did not attenuate mechanical allodynia. Spinal glial activation was unaltered by any treatment. These data provide further evidence for the role of central IL-6 and neuroimmune modulation in the etiology of mechanical allodynia following peripheral nerve injury.

Emerging cell and molecular strategies for the study and treatment of painful peripheral neuropathies

Pharmacologic treatment for the symptoms of painful neuropathy has been problematic, because there has been limited understanding of the underlying etiologies and systemic levels that an effective dose can have on multiple side effects. The use of molecular methods, such as gene deletion from knockout mice and cellular minipumps for delivery of biologic antinociceptive molecules, has led to a better understanding of the underlying mechanisms of the induction of intractable neuropathic pain. The initiation of an excitatory cascade after injury or disease leads to the induction of various second messenger systems, loss or down-regulation of the endogenous inhibitory spinal GABA system and central sensitization, causing such pain. The development and use of cellular minipumps, immortalized cell lines bioengineered to secrete various antinociceptive molecules for the reversal of neuropathic pain, makes cellular therapy a strategy for clinical use in the next few years. The development of molecular "disimmortalization" technologies will make the use of such engineered cell lines safe for human use. Direct somatic gene transfer for neuropathic pain will eventually overcome the problems associated with transplantation of non-autologous and xenogenic cells. These virus-mediated methods, although at the early stages of evolution and use, offer large-scale production of biologic agents that can be conveniently and confidently used for the long-term relief of chronic neuropathic pain in a clinical setting, without systemic effects or surgical interventions.

Reciprocal actions of interleukin-6 and brain-derived neurotrophic factor on rat and mouse primary sensory neurons

In low-density, serum-free cultures of neurons from embryonic rat dorsal root ganglia, interleukin-6 supports the survival of less than one third of the neurons yet virtually all of them bear interleukin-6 alpha-receptors. A finding that might explain this selectivity is that interleukin-6 acts on sensory neurons in culture through a mechanism requiring endogenous brain-derived neurotrophic factor. Antibodies or a trkB fusion protein that block the biological activity of brain-derived neurotrophic factor synthesized by dorsal root ganglion neurons also block the survival-promoting actions of interleukin-6 on these neurons. Two results indicate that interleukin-6 influences synthesis of brain-derived neurotrophic factor in adult dorsal root ganglion neurons. Intrathecal infusion of interleukin-6 in rats increases the concentration of brain-derived neurotrophic factor mRNA in rat lumbar dorsal root ganglia. The induction of brain-derived neurotrophic factor in dorsal root ganglion neurons that is seen after nerve injury in rats or wild-type mice is severely attenuated in mice with null mutation of the interleukin-6 gene. In brief, the ability of interleukin-6 to support the survival of embryonic sensory neurons in vitro depends upon the presence of endogenous brain-derived neurotrophic factor and the induction of brain-derived neurotrophic factor in injured adult sensory neurons depends upon the presence of endogenous interleukin-6.

Galanin knockout mice reveal nociceptive deficits following peripheral nerve injury

The neuropeptide galanin has been identified as a potential neurotransmitter/neuromodulator within the central nervous system. In the present study, the role of endogenous galanin in nociceptive processing in the nervous system has been analysed by using mice carrying a targeted mutation in the galanin gene. Supporting this, the effect of chronic administration of exogenous galanin on nociceptive sensory inputs has been assayed in adult rats. In the absence of peripheral nerve injury, the sensitivity to threshold noxious stimuli is significantly higher in galanin mutant mice than wild-type controls. Following peripheral nerve injury, in conditions under which endogenous galanin levels are elevated, spontaneous and evoked neuropathic pain behaviours are compromised in mutant mice. Conversely, chronic intrathecal delivery of exogenous galanin to nerve-intact adult rats is associated with persistent behavioural hypersensitivity, a significant increase in c-fos expression and an increase in PKCgamma immunoreactivity within the spinal cord dorsal horn. The present results demonstrate that a relationship exists between the degree of nerve injury-induced galanin expression and the degree of behavioural hypersensitivity, and show that galanin may play a role in nociceptive processing in the spinal cord, with interrelated inhibitory and excitatory effects.