IL-4 Deficiency Is Associated with Mechanical Hypersensitivity in Mice

Interleukin-4 (IL-4) is an anti-inflammatory and analgesic cytokine that induces opioid receptor transcription. We investigated IL-4 knockout (ko) mice to characterize their pain behavior before and after chronic constriction injury (CCI) of the sciatic nerve as a model for neuropathic pain. We investigated opioid responsivity and measured cytokine and opioid receptor gene expression in the peripheral and central nervous system (PNS, CNS) of IL-4 ko mice in comparison with wildtype (wt) mice. Naïve IL-4 ko mice displayed tactile allodynia (wt: 0.45 g; ko: 0.18 g; p<0.001), while responses to heat and cold stimuli and to muscle pressure were not different. No compensatory changes in the gene expression of tumor necrosis factor-alpha (TNF), IL-1β, IL-10, and IL-13 were found in the PNS and CNS of naïve IL-4 ko mice. However, IL-1β gene expression was stronger in the sciatic nerve of IL-4 ko mice (p<0.001) 28 days after CCI and only IL-4 ko mice had elevated IL-10 gene expression (p = 0.014). Remarkably, CCI induced TNF (p<0.01), IL-1β (p<0.05), IL-10 (p<0.05), and IL-13 (p<0.001) gene expression exclusively in the ipsilateral spinal cord of IL-4 ko mice. The compensatory overexpression of the anti-inflammatory and analgesic cytokines IL-10 and IL-13 in the spinal cord of IL-4 ko mice may explain the lack of genotype differences for pain behavior after CCI. Additionally, CCI induced gene expression of μ, κ, and δ opioid receptors in the contralateral cortex and thalamus of IL-4 ko mice, paralleled by fast onset of morphine analgesia, but not in wt mice. We conclude that a lack of IL-4 leads to mechanical sensitivity; the compensatory hyperexpression of analgesic cytokines and opioid receptors after CCI, in turn, protects IL-4 ko mice from enhanced pain behavior after nerve lesion

Analgesic effect of morphine in IL-4 ko and wt mice.

<p>Paw withdrawal latencies to thermal stimuli and withdrawal thresholds to von Frey hairs in wt and IL-4 ko mice after CCI (A, B) and additional morphine treatment (C, D). CCI leads to thermal (A) and mechanical (B) hypersensitivity lasting up to day 28 after CCI. Mice received morphine i.p. (abbreviated as M in the graphs) at day seven post-surgery. Only IL-4 ko mice showed elevation of mechanical withdrawal thresholds and prolongation thermal withdrawal latencies at 2 h after morphine i.p. (A, B). Asterisk: *p<0.05, **p<0.01, ***p<0.001.</p

Cytokine gene expression before and after CCI.

<p>The box- and whisker plots illustrate the relative gene expression of pro- and anti-inflammatory cytokines in the ipsilateral sciatic nerve (A), and ipsilateral lumbar spinal cord (B) 28 days after CCI. A) CCI leads to an increase in TNF and IL-1ß gene expression in the ipsilateral sciatic nerve. The increase in IL-1ß gene expression is higher in IL-4 ko mice compared to wt mice. IL-10 gene expression increases only in IL-4 ko mice. B) CCI leads to an increase in TNF, IL-1ß, IL-10, and IL-13 gene expression only in IL-4 ko mice; this increase is higher compared to wt mice after CCI (TNF: p = 0.002; IL-1ß: p<0.001; IL-10: p = 0.011; IL-13: p = 0.026). Asterisk: *p<0.05, **p<0.01, ***p<0.001.</p

Behavioral tests in naïve IL-4 ko and wt mice.

<p>The bars illustrate the results of the behavioral tests in naïve wt and IL-4 ko mice. A) IL-4 ko mice do not differ from wt mice in withdrawal latencies to heat. B) IL-4 ko mice have reduced paw withdrawal thresholds to mechanical stimulation with von-Frey filaments (***p<0.001). IL-4 ko mice do not differ from wt mice in paw withdrawal time to acetone (C), and in withdrawal latencies of the hind limb upon pressure to the gastrocnemius muscle (D).</p

Spinal opioid receptor gene expression in IL-4 ko and wt mice before and after CCI.

<p>The box- and whisker plots illustrate the relative gene expression of the opioid receptors MOR, DOR, and KOR in the lumbar spinal cord of naïve wt and IL-4 ko mice and at day 28 after CCI (data shown for contralateral spinal cord sections; also no changes in opioid receptor gene expression were found in the inpsilateral spinal cord (data not shown). No intergroup difference is found before and after nerve lesion.</p

Cerebral opioid receptor gene expression in IL-4 ko and wt mice before and after CCI.

<p>The box- and whisker plots illustrate the relative gene expression of the opioid receptors MOR (A), DOR (B), and KOR (C) in the contralateral frontal cortex and thalamus at day 28 after CCI. A) MOR gene expression decreases in the contralateral frontal cortex in IL-4 ko mice and in wt mice. B) DOR gene expression increases in the contralateral thalamus in both wt and IL-4 ko mice. C) KOR gene expression decreases in the contralateral frontal cortex of IL-4 ko and wt (n.s.) mice and increases in the contralateral thalamus only in IL-4 ko mice. Asterisk: *p<0.05, **p<0.01, ***p<0.001.</p